For foreign first-year students for autumn term Module Methodical elaboration for practice class on human anatomy for foreign first-year students for autumn term
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Development of the Teeth—In describing the development of the teeth, the mode of formation of the deciduous teeth must first be considered, and then that of the permanent series. Development of the Deciduous Teeth.—The development of the deciduous teeth begins about the sixth week of fetal life as a thickening of the epithelium along the line of the future jaw, the thickening being due to a rapid multiplication of the more deeply situated epithelial cells. As the cells multiply they extend into the subjacent mesoderm, and thus form a ridge or strand of cells imbedded in mesoderm. About the seventh week a longitudinal splitting or cleavage of this strand of cells takes place, and it becomes divided into two strands; the separation begins in front and extends laterally, the process occupying four or five weeks. Of the two strands thus formed, the labial forms the labiodental lamina; while the other, the lingual, is the ridge of cells in connection with which the teeth, both deciduous and permanent, are developed. Hence it is known as the dental lamina or common dental germ. It forms a flat band of cells, which grows into the substance of the embryonic jaw, at first horizontally inward, and then, as the teeth develop, vertically, i. e., upward in the upper jaw, and downward in the lower jaw. While still maintaining a horizontal direction it has two edges—an attached edge, continuous with the epithelium lining the mouth, and a free edge, projecting inward, and imbedded in the mesodermal tissue of the embryonic jaw. Along its line of attachment to the buccal epithelium is a shallow groove, the dental furrow. About the ninth week the dental lamina begins to develop enlargements along its free border. These are ten in number in each jaw, and each corresponds to a future deciduous tooth. They consist of masses of epithelial cells; and the cells of the deeper part—that is, the part farthest from the margin of the jaw—increase rapidly and spread out in all directions. Each mass thus comes to assume a club shape, connected with the general epithelial lining of the mouth by a narrow neck, embraced by mesoderm. They are now known as special dental germs. After a time the lower expanded portion inclines outward, so as to form an angle with the superficial constricted portion, which is sometimes known as the neck of the special dental germ. About the tenth week the mesodermal tissue beneath these special dental germs becomes differentiated into papillæ; these grow upward, and come in contact with the epithelial cells of the special dental germs, which become folded over them like a hood or cap. There is, then, at this stage a papilla (or papillæ) which has already begun to assume somewhat the shape of the crown of the future tooth, and from which the dentin and pulp of the tooth are formed, surmounted by a dome or cap of epithelial cells from which the enamel is derived. In the meantime, while these changes have been going on, the dental lamina has been extending backward behind the special dental germ corresponding to the second deciduous molar tooth, and at about the seventeenth week it presents an enlargement, the special dental germ, for the first permanent molar, soon followed by the formation of a papilla in the mesodermal tissue for the same tooth. This is followed, about the sixth month after birth, by a further extension backward of the dental lamina, with the formation of another enlargement and its corresponding papilla for the second molar. And finally the process is repeated for the third molar, its papilla appearing about the fifth year of life. After the formation of the special dental germs, the dental lamina undergoes atrophic changes and becomes cribriform, except on the lingual and lateral aspects of each of the special germs of the temporary teeth, where it undergoes a local thickening forming the special dental germ of each of the successional permanent teeth—i. e., the ten anterior ones in each jaw. Here the same process goes on as has been described in connection with those of the deciduous teeth: that is, they recede into the substance of the gum behind the germs of the deciduous teeth. As they recede they become club-shaped, form expansions at their distal extremities, and finally meet papillæ, which have been formed in the mesoderm, just in the same manner as was the case in the deciduous teeth. The apex of each papilla indents the dental germ, which encloses it, and, forming a cap for it, becomes converted into the enamel, while the papilla forms the dentin and pulp of the permanent tooth. The special dental germs consist at first of rounded or polyhedral epithelial cells; after the formation of the papillæ, these cells undergo a differentiation into three layers. Those which are in immediate contact with the papilla become elongated, and form a layer of well-marked columnar epithelium coating the papilla. They are the cells which form the enamel fibers, and are therefore termed enamel cells or adamantoblasts. The cells of the outer layer of the special dental germ, which are in contact with the inner surface of the dental sac, presently to be described, are much shorter, cubical in form, and are named the external enamel epithelium. All the intermediate round cells of the dental germ between these two layers undergo a peculiar change. They become stellate in shape and develop processes, which unite to form a net-work into which fluid is secreted; this has the appearance of a jelly, and to it the name of enamel pulp is given. This transformed special dental germ is now known under the name of enamel organ. While these changes are going on, a sac is formed around each enamel organ from the surrounding mesodermal tissue. This is known as the dental sac, and is a vascular membrane of connective tissue. It grows up from below, and thus encloses the whole tooth germ; as it grows it causes the neck of the enamel organ to atrophy and disappear; so that all communication between the enamel organ and the superficial epithelium is cut off. At this stage there are vascular papillæ surmounted by caps of epithelial cells, the whole being surrounded by by membranous sacs. Formation of the Enamel.—The enamel is formed exclusively from the enamel cells or adamantoblasts of the special dental germ, either by direct calcification of the columnar cells, which become elongated into the hexagonal rods of the enamel; or, as is more generally believed, as a secretion from the adamantoblasts, within which calcareous matter is subsequently deposited. The process begins at the apex of each cusp, at the ends of the enamel cells in contact with the dental papilla. Here a fine globular deposit takes place, being apparently shed from the end of the adamantoblasts. It is known by the name of the enamel droplet, and resembles keratin in its resistance to the action of mineral acids. This droplet then becomes fibrous and calcifies and forms the first layer of the enamel; a second droplet now appears and calcifies, and so on; successive droplets of keratin-like material are shed from the adamantoblasts and form successive layers of enamel, the adamantoblasts gradually receding as each layer is produced, until at the termination of the process they have almost disappeared. The intermediate cells of the enamel pulp atrophy and disappear, so that the newly formed calcified material and the external enamel epithelium come into apposition. This latter layer, however, soon disappears on the emergence of the tooth beyond the gum. After its disappearance the crown of the tooth is still covered by a distinct membrane, which persists for some time. This is known as the cuticula dentis, or Nasmyth’s membrane, and is believed to be the last-formed layer of enamel derived from the adamantoblasts, which has not become calcified. It forms a horny layer, which may be separated from the subjacent calcified mass by the action of strong acids. It is marked by the hexagonal impressions of the enamel prisms, and, when stained by nitrate of silver, shows the characteristic appearance of epithelium. Formation of the Dentin.—While these changes are taking place in the epithelium to form the enamel, contemporaneous changes occurring in the differentiated mesoderm of the dental papillæ result in the formation of the dentin. As before stated, the first germs of the dentin are the papillæ, corresponding in number to the teeth, formed from the soft mesodermal tissue which bounds the depressions containing the special enamel germs. The papillæ grow upward into the enamel germs and become covered by them, both being enclosed in a vascular connective tissue, the dental sac, in the manner above described. Each papilla then constitutes the formative pulp from which the dentin and permanent pulp are developed; it consists of rounded cells and is very vascular, and soon begins to assume the shape of the future tooth. The next step is the appearance of the odontoblasts, which have a relation to the development of the teeth similar to that of the osteoblasts to the formation of bone. They are formed from the cells of the periphery of the papilla—that is to say, from the cells in immediate contact with the adamantoblasts of the special dental germ. These cells become elongated, one end of the elongated cell resting against the epithelium of the special dental germs, the other being tapered and oftened branched. By the direct transformation of the peripheral ends of these cells; or by a secretion from them, a layer of uncalcified matrix (prodentin) is formed which caps the cusp or cusps, if there are more than one, of the papillæ. This matrix becomes fibrillated, and in it islets of calcification make their appearance, and coalescing give rise to a continuous layer of calcified material which covers each cusp and constitutes the first layer of dentin. The odontoblasts, having thus formed the first layer, retire toward the center of the papilla, and, as they do so, produce successive layers of dentin from their peripheral extremities—that is to say, they form the dentinal matrix in which calcification subsequently takes place. As they thus recede from the periphery of the papilla, they leave behind them filamentous processes of cell protoplasm, provided with finer side processes; these are surrounded by calcified material, and thus form the dental canaliculi, and, by their side branches, the anastomosing canaliculi: the processes of protoplasm contained within them constitute the dentinal fibers (Tomes’ fibers). In this way the entire thickness of the dentin is developed, each canaliculus being completed throughout its whole length by a single odontoblast. The central part of the papilla does not undergo calcification, but persists as the pulp of the tooth. In this process of formation of dentin it has been shown that an uncalcified matrix is first developed, and that in this matrix islets of calcification appear which subsequently blend together to form a cap to each cusp: in like manner successive layers are produced, which ultimately become blended with each other. In certain places this blending is not complete, portions of the matrix remaining uncalcified between the successive layers; this gives rise to little spaces, which are the interglobular spaces alluded to above. Formation of the Cement.—The root of the tooth begins to be formed shortly before the crown emerges through the gum, but is not completed until some time afterward. It is produced by a downgrowth of the epithelium of the dental germ, which extends almost as far as the situation of the apex of the future root, and determines the form of this portion of the tooth. This fold of epithelium is known as the epithelial sheath, and on its papillary surface odontoblasts appear, which in turn form dentin, so that the dentin formation is identical in the crown and root of the tooth. After the dentin of the root has been developed, the vascular tissues of the dental sac begin to break through the epithelial sheath, and spread over the surface of the root as a layer of bone-forming material. In this osteoblasts make their appearance, and the process of ossification goes on in identically the same manner as in the ordinary intramembranous ossification of bone. In this way the cement is formed, and consists of ordinary bone containing canaliculi and lacunæ. Formation of the Alveoli.—About the fourteenth week of embryonic life the dental lamina becomes enclosed in a trough or groove of mesodermal tissue, which at first is common to all the dental germs, but subsequently becomes divided by bony septa into loculi, each loculus containing the special dental germ of a deciduous tooth and its corresponding permanent tooth. After birth each cavity becomes subdivided, so as to form separate loculi (the future alveoli) for the deciduous tooth and its corresponding permanent tooth. Although at one time the whole of the growing tooth is contained in the cavity of the alveolus, the latter never completely encloses it, since there is always an aperture over the top of the crown filled by soft tissue, by which the dental sac is connected with the surface of the gum, and which in the permanent teeth is called the gubernaculum dentis.
The superadded permanent teeth are developed in the manner already described, by extensions backward of the posterior part of the dental lamina in each jaw. Eruption of the Teeth.—When the calcification of the different tissues of the tooth is sufficiently advanced to enable it to bear the pressure to which it will be afterward subjected, eruption takes place, the tooth making its way through the gum. The gum is absorbed by the pressure of the crown of the tooth against it, which is itself pressed up by the increasing size of the root. At the same time the septa between the dental sacs ossify, and constitute the alveoli; these firmly embrace the necks of the teeth, and afford them a solid basis of support. The eruption of the deciduous teeth commences about the seventh month after birth, and is completed about the end of the second year, the teeth of the lower jaw preceding those of the upper. The following, according to C. S. Tomes, are the most usual times of eruption: Lower central incisors 6 to 9 months. Upper incisors 8 to 10 months. Lower lateral incisors and first molars 15 to 21 months. Canines
16 to 20 months. Second molars 20 to 24 months.
At the age of 1 year a child should have 6 teeth. At the age of 1 1/2 years a child should have 12 teeth. At the age of 2 year a child should have 16 teeth. At the age of 2 1/2 years a child should have 20 teeth. Calcification of the permanent teeth proceeds in the following order in the lower jaw (in the upper jaw it takes place a little later): the first molar, soon after birth; the central and lateral incisors, and the canine, about six months after birth; the premolars, at the second year, or a little later; the second molar, about the end of the second year; the third molar, about the twelfth year. The eruption of the permanent teeth takes place at the following periods, the teeth of the lower jaw preceding those of the upper by short intervals: First molars 6th year. Two central incisors 7th year. Two lateral incisors 8th year. First premolars 9th year. Second premolars 10th year. Canines
11th to 12th year. Second molars 12th to 13th year. Third molars 17th to 25th year. Toward the sixth year, before the shedding of the deciduous teeth begins, there are twenty-four teeth in each jaw, viz., the ten deciduous teeth and the crowns of all the permanent teeth except the third molars. Parotid Gland (glandula parotis).—The parotid gland, the largest of the three, varies in weight from 14 to 28 gm. It lies upon the side of the face, immediately below and in front of the external ear. The main portion of the gland is superficial, somewhat flattened and quadrilateral in form, and is placed between the ramus of the mandible in front and the mastoid process and Sternocleidomastoideus behind, overlapping, however, both boundaries. Above, it is broad and reaches nearly to the zygomatic arch; below, it tapers somewhat to about the level of a line joining the tip of the mastoid process to the angle of the mandible. The remainder of the gland is irregularly wedge-shaped, and extends deeply inward toward the pharyngeal wall. The gland is enclosed within a capsule continuous with the deep cervical fascia; the layer covering the superficial surface is dense and closely adherent to the gland; a portion of the fascia, attached to the styloid process and the angle of the mandible, is thickened to form the stylomandibular ligament which intervenes between the parotid and submaxillary glands. The anterior surface of the gland is moulded on the posterior border of the ramus of the mandible, clothed by the Pterygoideus internus and Masseter. The inner lip of the groove dips, for a short distance, between the two Pterygoid muscles, while the outer lip extends for some distance over the superficial surface of the Masseter; a small portion of this lip immediately below the zygomatic arch is usually detached, and is named the accessory part (socia parotidis) of the gland. The posterior surface is grooved longitudinally and abuts against the external acoustic meatus, the mastoid process, and the anterior border of the Sternocleidomastoideus. The superficial surface, slightly lobulated, is covered by the integument, the superficial fascia containing the facial branches of the great auricular nerve and some small lymph glands, and the fascia which forms the capsule of the gland. The deep surface extends inward by means of two processes, one of which lies on the Digastricus, styloid process, and the styloid group of muscles, and projects under the mastoid process and Sternocleidomastoideus; the other is situated in front of the styloid process, and sometimes passes into the posterior part of the mandibular fossa behind the temporomandibular joint. The deep surface is in contact with the internal and external carotid arteries, the internal jugular vein, and the vagus and glossopharyngeal nerves. The gland is separated from the pharyngeal wall by some loose connective tissue.
The parotid duct (ductus parotideus; Stensen’s duct) is about 7 cm. long. It begins by numerous branches from the anterior part of the gland, crosses the Masseter, and at the anterior border of this muscle turns inward nearly at a right angle, passes through the corpus adiposum of the cheek and pierces the Buccinator; it then runs for a short distance obliquely forward between the Buccinator and mucous membrane of the mouth, and opens upon the oral surface of the cheek by a small orifice, opposite the second upper molar tooth. While crossing the Masseter, it receives the duct of the accessory portion; in this position it lies between the branches of the facial nerve; the accessory part of the gland and the transverse facial artery are above it. Structure.—The parotid duct is dense, its wall being of considerable thickness; its canal is about the size of a crow-quill, but at its orifice on the oral surface of the cheek its lumen is greatly reduced in size. It consists of a thick external fibrous coat which contains contractile fibers, and of an internal or mucous coat lined with short columnar epithelium. Vessels and Nerves.—The arteries supplying the parotid gland are derived from the external carotid, and from the branches given off by that vessel in or near its substance. The veins empty themselves into the external jugular, through some of its tributaries. The lymphatics end in the superficial and deep cervical lymph glands, passing in their course through two or three glands, placed on the surface and in the substance of the parotid. The nerves are derived from the plexus of the sympathetic on the external carotid artery, the facial, the auriculotemporal, and the great auricular nerves. It is probable that the branch from the auriculotemporal nerve is derived from the glossopharyngeal through the otic ganglion. At all events, in some of the lower animals this has been proved experimentally to be the case. Submaxillary Gland (glandula submaxillaris).—The submaxillary gland is irregular in form and about the size of a walnut. A considerable part of it is situated in the submaxillary triangle, reaching forward to the anterior belly of the Digastricus and backward to the stylomandibular ligament, which intervenes between it and the parotid gland. Above, it extends under cover of the body of the mandible; below, it usually overlaps the intermediate tendon of the Digastricus and the insertion of the Stylohyoideus, while from its deep surface a tongue-like deep process extends forward above the Mylohyoideus muscle. Its superficial surface consists of an upper and a lower part. The upper part is directed outward, and lies partly against the submaxillary depression on the inner surface of the body of the mandible, and partly on the Pterygoideus internus. The lower part is directed downward and outward, and is covered by the skin, superficial fascia, Platysma, and deep cervical fascia; it is crossed by the anterior facial vein and by filaments of the facial nerve; in contact with it, near the mandible, are the submaxillary lymph glands. The deep surface is in relation with the Mylohyoideus, Hyoglossus, Styloglossus, Stylohyoideus, and posterior belly of the Digastricus; in contact with it are the mylohyoid nerve and the mylohyoid and submental vessels. The external maxillary artery is imbedded in a groove in the posterior border of the gland. The deep process of the gland extends forward between the Mylohyoideus below and externally, and the Hyoglossus and Styloglossus internally; above it is the lingual nerve and submaxillary ganglion; below it the hypoglossal nerve and its accompanying vein. The submaxillary duct (ductus submaxillaris; Wharton’s duct) is about 5 cm. long, and its wall is much thinner than that of the parotid duct. It begins by numerous branches from the deep surface of the gland, and runs forward between the Mylohyoideus and the Hyoglossus and Genioglossus, then between the sublingual gland and the Genioglossus, and opens by a narrow orifice on the summit of a small papilla, at the side of the frenulum linguæ. On the Hyoglossus it lies between the lingual and hypoglossal nerves, but at the anterior border of the muscle it is crossed laterally by the lingual nerve; the terminal branches of the lingual nerve ascend on its medial side. Vessels and Nerves.—The arteries supplying the submaxillary gland are branches of the external maxillary and lingual. Its veins follow the course of the arteries. The nerves are derived from the submaxillary ganglion, through which it receives filaments from the chorda tympani of the facial nerve and the lingual branch of the mandibular, sometimes from the mylohyoid branch of the inferior alveolar, and from the sympathetic. Sublingual Gland (glandula sublingualis).—The sublingual gland is the smallest of the three glands. It is situated beneath the mucous membrane of the floor of the mouth, at the side of the frenulum linguæ, in contact with the sublingual depression on the inner surface of the mandible, close to the symphysis. It is narrow, flattened, shaped somewhat like an almond, and weighs nearly 2 gm. It is in relation, above, with the mucous membrane; below, with the Mylohyoideus; behind, with the deep part of the submaxillary gland; laterally, with the mandible; and medially, with the Genioglossus, from which it is separated by the lingual nerve and the submaxillary duct. Its excretory ducts are from eight to twenty in number. Of the smaller sublingual ducts (ducts of Rivinus), some join the submaxillary duct; others open separately into the mouth, on the elevated crest of mucous membrane (plica sublingualis), caused by the projection of the gland, on either side of the frenulum linguæ. One or more join to form the larger sublingual duct (duct of Bartholin), which opens into the submaxillary duct. Vessels and Nerves.—The sublingual gland is supplied with blood from the sublingual and submental arteries. Its nerves are derived from the lingual, the chorda tympani, and the sympathetic. Structure of the Salivary Glands.—The salivary glands are compound racemose glands, consisting of numerous lobes, which are made up of smaller lobules, connected together by dense areolar tissue, vessels, and ducts. Each lobule consists of the ramifications of a single duct, the branches ending in dilated ends or alveoli on which the capillaries are distributed. The alveoli are enclosed by a basement membrane, which is continuous with the membrana propria of the duct and consists of a net-work of branched and flattened nucleated cells. The alveoli of the salivary glands are of two kinds, which differ in the appearance of their secreting cells, in their size, and in the nature of their secretion. (1) The mucous variety secretes a viscid fluid, which contains mucin; (2) the serous variety secretes a thinner and more watery fluid. The sublingual gland consists of mucous, the parotid of serous alveoli. The submaxillary contains both mucous and serous alveoli, the latter, however, preponderating. The cells in the mucous alveoli are columnar in shape. In the fresh condition they contain large granules of mucinogen. In hardened preparations a delicate protoplasmic net-work is seen, and the cells are clear and transparent. The nucleus is usually situated near the basement membrane, and is flattened. In some alveoli are seen peculiar crescentic bodies, lying between the cells and the membrana propria. They are termed the crescents of Gianuzzi, or the demilunes of Heidenhain and are composed of polyhedral granular cells, which Heidenhain regards as young epithelial cells destined to supply the place of those salivary cells which have undergone disintegration. This view, however, is not accepted by Klein. Fine canaliculi pass between the mucus-secreting cells to reach the demilunes and even penetrate the cells forming these structures. In the serous alveoli the cells almost completely fill the cavity, so that there is hardly any lumen perceptible; they contain secretory granules imbedded in a closely reticulated protoplasm The cells are more cubical than those of the mucous type; the nucleus of each is spherical and placed near the center of the cell, and the granules are smaller. Both mucous and serous cells vary in appearance according to whether the gland is in a resting condition or has been recently active. In the former case the cells are large and contain many secretory granules; in the latter case they are shrunken and contain few granules, chiefly collected at the inner ends of the cells. The granules are best seen in fresh preparations. The ducts are lined at their origins by epithelium which differs little from the pavement form. As the ducts enlarge, the epithelial cells change to the columnar type, and the part of the cell next the basement membrane is finely striated. The lobules of the salivary glands are richly supplied with bloodvessels which form a dense net-work in the interalveolar spaces. Fine plexuses of nerves are also found in the interlobular tissue. The nerve fibrils pierce the basement membrane of the alveoli, and end in branched varicose filaments between the secreting cells. In the hilus of the submaxillary gland there is a collection of nerve cells termed Langley’s ganglion. Accessory Glands.—Besides the salivary glands proper, numerous other glands are found in the mouth. Many of these glands are found at the posterior part of the dorsum of the tongue behind the vallate papillæ, and also along its margins as far forward as the apex. Others lie around and in the palatine tonsil between its crypts, and large numbers are present in the soft palate, the lips, and cheeks. These glands are of the same structure as the larger salivary glands, and are of the mucous or mixed type. 7. Methodic of class work: a) interrogation of the students on the home task; b) study of samples (topic according to the plan); c) fill in the protocol of current lesson; d) checking and signing the protocols by teacher.
Questions: Situational tasks: Tests. 9. The illustrative material: tables, samples. 10. Sources of the information: Human anatomy 11. The program of self-preparation of students: 1. To learn the appropriate sections under the textbook 2. To consider preparations and to study them according to the plan of practical class. 3. To fill in the report of practical class. 4. To be able to show on a preparation of the Teeth, the Fauces, the salivary glands.
2. The place: classroom of the department of human anatomy. 3. The aim: to know the topography and structure of the pharynx, the esophagus, the stomach and the small intestine. 4. The professional orientation of students: The knowledge of this topic are necessary for doctors of all specialities, it represents special interest for therapists. 5. The basic of knowledge: 6. The plan of the practice class: A. Checking of the home task: interrogation or the test control – 30 min B. Summary lecture on the topic by teacher – 20 min а) the pharynx; b) the esophagus; c) the stomach; d) the small intestine. C. Self-taught class– 100 min Working plan: The Pharynx The pharynx is that part of the digestive tube which is placed behind the nasal cavities, mouth, and larynx. It is a musculomembranous tube, somewhat conical in form, with the base upward, and the apex downward, extending from the under surface of the skull to the level of the cricoid cartilage in front, and that of the sixth cervical vertebra behind. The cavity of the pharynx is about 12.5 cm. long, and broader in the transverse than in the antero-posterior diameter. Its greatest breadth is immediately below the base of the skull, where it projects on either side, behind the pharyngeal ostium of the auditory tube, as the pharyngeal recess (fossa of Rosenmüller); its narrowest point is at its termination in the esophagus. It is limited, above, by the body of the sphenoid and basilar part of the occipital bone; below, it is continuous with the esophagus; posteriorly, it is connected by loose areolar tissue with the cervical portion of the vertebral column, and the prevertebral fascia covering the Longus colli and Longus capitis muscles; anteriorly, it is incomplete, and is attached in succession to the medial pterygoid plate, pterygomandibular raphé, mandible, tongue, hyoid bone, and thyroid and cricoid cartilages; laterally, it is connected to the styloid processes and their muscles, and is in contact with the common and internal carotid arteries, the internal jugular veins, the glossopharyngeal, vagus, and hypoglossal nerves, and the sympathetic trunks, and above with small parts of the Pterygoidei interni. Seven cavities communicate with it, viz., the two nasal cavities, the two tympanic cavities, the mouth, the larynx, and the esophagus. The cavity of the pharynx may be subdivided from above downward into three parts: nasal, oral, and laryngeal. The Nasal Part of the Pharynx (pars nasalis pharyngis; nasopharynx) lies behind the nose and above the level of the soft palate: it differs from the oral and laryngeal parts of the pharynx in that its cavity always remains patent. In front it communicates through the choanæ with the nasal cavities. On its lateral wall is the pharyngeal ostium of the auditory tube, somewhat triangular in shape, and bounded behind by a firm prominence, the torus or cushion, caused by the medial end of the cartilage of the tube which elevates the mucous membrane. A vertical fold of mucous membrane, the salpingopharyngeal fold, stretches from the lower part of the torus; it contains the Salpingopharyngeus muscle. A second and smaller fold, the salpingopalatine fold, stretches from the upper part of the torus to the palate. Behind the ostium of the auditory tube is a deep recess, the pharyngeal recess (fossa of Rosenmüller). On the posterior wall is a prominence, best marked in childhood, produced by a mass of lymphoid tissue, which is known as the pharyngeal tonsil. Above the pharyngeal tonsil, in the middle line, an irregular flask-shaped depression of the mucous membrane sometimes extends up as far as the basilar process of the occipital bone; it is known as the pharyngeal bursa. The Oral Part of the Pharynx (pars oralis pharyngis) reaches from the soft palate to the level of the hyoid bone. It opens anteriorly, through the isthmus faucium, into the mouth, while in its lateral wall, between the two palatine arches, is the palatine tonsil. The Laryngeal Part of the Pharynx (pars laryngea pharyngis) reaches from the hyoid bone to the lower border of the cricoid cartilage, where it is continuous with the esophagus. In front it presents the triangular entrance of the larynx, the base of which is directed forward and is formed by the epiglottis, while its lateral boundaries are constituted by the aryepiglottic folds. On either side of the laryngeal orifice is a recess, termed the sinus piriformis, which is bounded medially by the aryepiglottic fold, laterally by the thyroid cartilage and hyothyroid membrane. Muscles of the Pharynx.—The muscles of the pharynx are: Constrictor inferior. Stylopharyngeus. Constrictor medius. Salpingopharyngeus. Constrictor superior. Pharyngopalatinus. The Constrictor pharyngis inferior (Inferior constrictor), the thickest of the three constrictors, arises from the sides of the cricoid and thyroid cartilage. From the cricoid cartilage it arises in the interval between the Cricothyreoideus in front, and the articular facet for the inferior cornu of the thyroid cartilage behind. On the thyroid cartilage it arises from the oblique line on the side of the lamina, from the surface behind this nearly as far as the posterior border and from the inferior cornu. From these origins the fibers spread backward and medialward to be inserted with the muscle of the opposite side into the fibrous raphé in the posterior median line of the pharynx. The inferior fibers are horizontal and continuous with the circular fibers of the esophagus; the rest ascend, increasing in obliquity, and overlap the Constrictor medius. The Constrictor pharyngis medius (Middle constrictor) is a fanshaped muscle, smaller than the preceding. It arises from the whole length of the upper border of the greater cornu of the hyoid bone, from the lesser cornu, and from the stylohyoid ligament. The fibers diverge from their origin: the lower ones descend beneath the Constrictor inferior, the middle fibers pass transversely, and the upper fibers ascend and overlap the Constrictor superior. It is inserted into the posterior median fibrous raphé, blending in the middle line with the muscle of the opposite side. The Constrictor pharyngis superior (Superior constrictor) is a quadrilateral muscle, thinner and paler than the other two. It arises from the lower third of the posterior margin of the medial pterygoid plate and its hamulus, from the pterygomandibular raphé, from the alveolar process of the mandible above the posterior end of the mylohyoid line, and by a few fibers from the side of the tongue. The fibers curve backward to be inserted into the median raphé, being also prolonged by means of an aponeurosis to the pharyngeal spine on the basilar part of the occipital bone. The superior fibers arch beneath the Levator veli palatini and the auditory tube. The interval between the upper border of the muscle and the base of the skull is closed by the pharyngeal aponeurosis, and is known as the sinus of Morgagni. The Stylopharyngeus is a long, slender muscle, cylindrical above, flattened below. It arises from the medial side of the base of the styloid process, passes downward along the side of the pharynx between the Constrictores superior and medius, and spreads out beneath the mucous membrane. Some of its fibers are lost in the Constrictor muscles, while others, joining with the Pharyngopalatinus, are inserted into the posterior border of the thyroid cartilage. The glossopharyngeal nerve runs on the lateral side of this muscle, and crosses over it to reach the tongue. The Salpingopharyngeus arises from the inferior part of the auditory tube near its orifice; it passes downward and blends with the posterior fasciculus of the Pharyngopalatinus. Nerves.—The Constrictores and Salpingopharyngeus are supplied by branches from the pharyngeal plexus, the Constrictor inferior by additional branches from the external laryngeal and recurrent nerves, and the Stylopharyngeus by the glossopharyngeal nerve. Actions.—When deglutition is about to be performed, the pharynx is drawn upward and dilated in different directions, to receive the food propelled into it from the mouth. The Stylopharyngei, which are much farther removed from one another at their origin than at their insertion, draw the sides of the pharynx upward and lateralward, and so increase its transverse diameter; its breadth in the antero-posterior direction is increased by the larynx and tongue being carried forward in their ascent. As soon as the bolus of food is received in the pharynx, the elevator muscles relax, the pharynx descends, and the Constrictores contract upon the bolus, and convey it downward into the esophagus. Structure.—The pharynx is composed of three coats: mucous, fibrous, and muscular. The pharyngeal aponeurosis, or fibrous coat, is situated between the mucous and muscular layers. It is thick above where the muscular fibers are wanting, and is firmly connected to the basilar portion of the occipital and the petrous portions of the temporal bones. As it descends it diminishes in thickness, and is gradually lost. It is strengthened posteriorly by a strong fibrous band, which is attached above to the pharyngeal spine on the under surface of the basilar portion of the occipital bone, and passes downward, forming a median raphé, which gives attachment to the Constrictores pharyngis. The mucous coat is continuous with that lining the auditory tubes, the nasal cavities, the mouth, and the larynx. In the nasal part of the pharynx it is covered by columnar ciliated epithelium; in the oral and laryngeal portions the epithelium is stratified squamous. Beneath the mucous membrane are found racemose mucous glands; they are especially numerous at the upper part of the pharynx around the orifices of the auditory tubes.
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