VERTEBRATES. 41 



structural feature for the systematic zoologist, especially in the case of the mammals, 

 that a full description of the dental system of the different groups will be found under 

 these. The student will thus be able to test the law of gradual specialization to which 

 reference has just been made, and will find that it applies within the smaller grouj)s, as 

 also in proceeding from the lower to the higher classes. 



The second of the series of organs enumerated above — the tongue — is to be re- 

 garded as a fold of the mucous membrane of the floor of the mouth, supported by the 

 basihyal bone, or its anterior glossohyal process. In most fishes it is nothing more, but 

 in Amphibia and the higher classes, muscular tissue enters into its formation, and it be- 

 comes functional in securing the food, and moving it about within the mouth cavity. 

 The muscles are partly confined to the fold (intrinsic), and partly attached to the hy- 

 oid skeleton (extrinsic), but both form the great bulk of the tongue, comparatively 

 little glandular tissue entering into its formation. It was observed above that the end 

 buds, which are more uniformly distributed in fishes, are confined to the cavity of the 

 mouth in higher forms ; the tongue is their usual seat in mammals, where they con- 

 stitute the organ of taste, and are arranged on papillae of peculiar form. 



In shape the tongue is subject to much variation ; it is sometimes developed equally 

 in all directions round its surface of attachment, but its free portion generally extends 

 either forwards or backwards from that point. Thus, in the frogs, the tongue is attached 

 anteriorly, and its posterior bifid end is that which is thrust rapidly out in securing 

 insects. Again, in mammals, it is attached behind and free in front. Frequently thick 

 and fleshy in appearance, it is in many forms slender and cylindrical ; generally cov- 

 ered -with a soft mucous surface, it becomes, in the birds, coated with a horny layer, in 

 adaptation to entirely different uses. Its highest degree of specialization in one 

 direction is reached in man, where it enters into important functional relations to the 

 organs of voice and speech. 



The glands which open into the cavity of the mouth, secreting the saliva for ad- 

 mixture with the food, are not represented in the fishes, although they are present 

 throughout all of the higher groups. In the Amphibia and some reptiles they form 

 more or less continuous tracts, which, however, become restricted to particular spots 

 in the higher reptiles, birds, and mammals. The chief of these are the submaxillary 

 and sublingual glands in the floor of the mouth, the buccal and parotid in the lateral 

 walls. The secretions furnished by these glands are of two kinds, and are formed 

 by cells of different histological character, either they facilitate the swallowing of the 

 food, or they act upon it chemically ; but both forms of gland-cells may be united in 

 the same gland. Special functions are sometimes met with, as in the case of the ant- 

 eaters, where the submaxillary glands furnish the sticky secretion which aids the 

 tongue in securing their insect food, or in case of the venomous snakes, where 

 the parotid gland becomes an organ of offence and defence, being converted into the 

 poison-gland. A similar change of function of the ordinary cutaneous glands we have 

 met with in the group of the Amphibia. 



Before proceeding to describe the divisions of the alimentary canal, a short ac- 

 count of its structure may be useful. Various coats are distinguished, of which the 

 outermost (which looks into the coelom) is the pleuro-peritoneal membrane. Imme- 

 diately within this is the muscular coat, composed of two layers of generally unstriped 

 fibres which are disposed longitudinally in the outer, but circularly in the inner layer. 

 The muscular coat is connected with the innermost mucous coat by a submucous layer 

 of connective tissue, which contains the larger blood vessels and nerves. The smaller 



