1106 DIGESTIVE SYSTEM 



The development of the palate. — The hard and soft palates arise (fig. 853) in two ridges 

 of tissue, designated the palate shelves, which develop on the inner surfaces of the maxillary 

 processes. These shelves grow toward the median line, and at the beginning of the third 

 month of foetal life meet beneath the nasal septum, uniting with each other and with the nasal 

 septum, the union taking place from before backward. The incisive foramen indicates the 

 place of meeting of the premaxillary and palate shelves, which closes the primitive communi- 

 cation between the oral and the nasal cavity. A want of union of the palate shelves presents 

 an arrest of development known as cleft-palate. The uvula is similarly formed by the union of 

 the posterior ends of the lateral palate anlages, and a failure to unite may produce a bifid uvula. 

 The transverse palatine ridges are better developed in the infant than in the adult, and may 

 assist in holding the nipple in sucking. 



Variations. — Cleft-palate and bifid uvula were mentioned above. The transverse palatine 

 ridges are quite variable in number and prominence. On each side of the incisive papilla 

 there is often found a small pit or shallow tube, a vestige of the embryonal incisive canal (Mer- 

 kel). Sometimes there is instead a single median pit, representing the lower end of the incisive 

 (Stenson's) canal. These pits are remnants of the primitive embryonic communication between 

 mouth and nasal cavities. 



Comparative. — The palate is absent in fishes and amphibia, the choanse opening directly 

 into the primitive mouth cavity. In some birds, the palate shelves fail to unite, leaving a normal 

 cleft-palate. The incisive (Stenson's) canal remains open permanently in some mammals 

 (e. g., ruminants), bifurcating above and thus placing the mouth cavity in communication 

 with the nasal cavity on each side in the vicinity of Jacobson's organ. The transverse palatine 

 ridges are much better developed among many mammals, especially the carnivora. 



THE TONGUE 



The tongue [lingua] is a muscular organ covered with mucous membrane and 

 located in the floor of the mouth. It is an important organ of mastication, deglu- 

 tition, taste and speech. Upon its upper surface (figs. 854, 864) is a V-shaped 

 groove (sulcus terminalis) indicating the division of the tongue into two parts. 

 The larger anterior part, or body [corpus linguae] belongs to the floor of the mouth, 

 while the smaller posterior part, or root [radix linguse], forms the anterior wall of 

 the oral pharynx. The inferior surface (facies inferior) of the tongue is chiefly 

 attached to the muscles of the floor of the mouth, from the hyoid bone to the man- 

 dible (fig. 858). Anteriorly and laterally, however, the inferior surface of the 

 body is free and covered with mucosa. The superior surface of the body is called 

 the dorsum. It is separated from the inferior surface by the lateral 7nargins, 

 which meet anteriorly at the tip [apex linguae]. 



The dorsum of the tongue usually presents a slight median groove [sulcus medi- 

 anus linguae]. Its posterior end corresponds to a small pit of variable depth, the 

 foramen coBCum, which is placed at the apex of the V-shaped terminal sulcus, 

 The dorsum of the body has a characteristic rough appearance due to numerous 

 small projections, the lingual papillce. 



Lingual papillae. — Five or six varieties of papillae are distinguished, between which inter- 

 mediate forms occur. The conical [papillae conices] and thread-like [papillae filiformes] are 

 most numerous, and are arranged more or less distinctly in rows parallel with the terminal 

 sulcus (fig. 856). They are best developed toward the mid-line of the dorsum in its posterior 

 part. As shown in vertical section (fig. 856), each papilla consists of an axial core of vascular 

 fibrous tissue (from the lamina propria) often beset with smaller secondary papillae. The 

 stratified squamous epithelial covering often presents numerous thread-like prolongations from 

 the apex of the papilla. The papillae vary from 1 to 3 mm. in length. 



The fungiform ("toad-stool shaped") papillae are somewhat similar to the conical in struc- 

 ture, but larger and more prominent, with an expanded free portion and a slightly constricted 

 stalk of attachment. They are relatively few in number and are scattered irregularly over 

 the dorsum, being most numerous near the margins (fig. 864). They are easily distinguished 

 in life by their larger size and reddish colour. A smaller, flattened variety of the fungiform is 

 sometimes called the lenticular ('lens-shaped') papilla>. (This term, however, is applied by 

 Toldt to certain small rounded elevations with underlying lymphatic nodules in the mucosa of 

 the root of the tongue.) 



The vallate (circumvallate) papillae, usually seven to eleven in number, are conspicuous 

 and arranged in a V-shaped line parallel with and slightly anterior to the sulcus terminalis, 

 (figs. 854, 857). They are, as a rule, .shaped like sliort cylinders, 1 to 2 nun. in width, and 

 somewhat less in height. As is shown in section (fig. 857), each is surrounded by a trench or 

 fossa, into the bottom of which ojjen ducts of the .serous glands of von Ebner. On the sides of 

 the fossa' are the taste-buds, as described in the section on Sknse Organs. 



The foliate j)a[)ilhe are repres(;nt(!d by a few (five to eight) parallel transverse or vertical 

 folds of muco.sa, along the margins of the tongue just anterior to the glosso-palatine arch on 

 each side (fig. 864). They are variable in size and Konietimes rudimentary. In structure they 

 somewhat resemble the vallate paj)illa! (though of different form), their walls being studded 

 with taste-buds. 



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