ORGANS OF DIGESTION. 405 



limits of the urethro- sexual canal (C. d.) which receives the 

 openings of the two ureters (C././.) and the two unequal 

 oviducts (C. g. g.< h.} are also perceptible ; and on the me- 

 dian plain, in the dorsal part of the preputial cavity, is the 

 opening of the Bursa Fabricii (C. e.), the analogue of Cow- 

 per's glands, and terminating in the same part as these 

 glands in the mammalia. There being no placental attach- 

 ment of the embryo of birds, the cloacal part of their ali- 

 mentary canal is not extended outwards to form an allantois 

 as it is in the embryos of quadrupeds. 



XXIII. Mammalia. The digestive organs, like the gene- 

 ral form, the internal structure, and the living habits of the 

 species, vary more in the mammiferous animals than in any 

 other vertebrated class, and they present the highest type of 

 development in the various organs connected with this func- 

 tion -j but the different forms of their alimentary cavities are 

 most intimately connected with those of the masticating and 

 prehensile organs, the organs of locomotion and perception, 

 and more or less with all the other external and internal parts 

 of their economy. The high development of their organs of 

 sense, and the flexibility and softness of their sensitive lips and 

 tongue, enable these animals to perceive minuter differences 

 in the chemical and physical properties of their food. The 

 solid structure of their teeth, and their fixed condition in 

 deep alveoli of immoveable maxillary and intermaxillary 

 bones, enable them more effectually to disintegrate their 

 aliment, and to mix it with the secretions of the parotid, 

 submaxillary, and sublingual glands, which are rarely deficient 

 in this class. The teeth, formed in cutaneous sacs and suc- 

 cessively renewed during life, are disposed in single rows, 

 and have their forms regulated chiefly by the motions of the 

 jaws which support them, and their texture by the nature of 

 the food. They are rarely deficient as in the bird-like jaws of 

 the manis, myrmecophaya and echidna ; their place is some- 

 times supplied by horny laminae as in the ornithorhyncus and 

 the whale; sometimes they are simple, alternate, similarly 

 formed, prehensile cones, as in several of the cetacea, but 

 most generally they are distinguished by their forms and 

 positions into incisors, canine and molar teeth, of which the 

 last are most characteristic of the nature of the food. The 

 molar teeth are renewed eight times in the elephant, the in- 



