294 



COMPARATIVE ANATOMY 



NASAL ocvmr 



PALATE 

 MOUTH 



TONGUE 



NASAL PHARYNX 



proper separates the alimentary canal from the body wall. In annelids 

 as in all the higher animals there is no connexion between the two cavities, 

 enteron and coelom. The single tube that forms the body of lower forms 

 has become double, and the muscular activities of the alimentary canal 

 are carried on independently of those of the body wall. 



Among the forms which He near the main line of human ancestry, 

 pharynx, esophagus, and stomach are first differentiated in urochordates. 



A liver arises in the cephalo- 

 chordates. What is possibly a new 

 mouth, not homologous with the 

 mouth of amphioxus, appears in 

 cyclostomes. But this mouth is 

 still a sucking one, without jaws. 

 The cyclostomes contribute also a 

 pancreas and a bilobed liver. 



Elasmobranchs, utilizing dermal 

 scales as teeth and transforming a 

 visceral arch into a jaw, convert 

 the sucking mouth into a biting 

 one. They develop also a new 

 cavity, the cloaca, to receive the 

 wastes and secretions of the 

 urogenital and digestive systems. 



The amphibia fasten their teeth 

 in a groove in the jaw bone, invent 

 salivary glands, utilize hypo- 

 branchial muscles to make a mobile 

 tongue, and differentiate small from 

 large intestine. 



Mammals greatly elongate the 

 intestine, and by suppressing the 

 cloaca separate the rectum from the urogenital sinus. The result is a 

 muscular, epithelium-lined alimentary canal, differentiated into nearly a 

 dozen different organs, and having about the same number of different 

 glands associated with it. 



Fig. 246. — Diagram of the alimsntary canal 

 (From Morris' " Human Anatomy.") 



THE HUMAN DIGESTIVE SYSTEM 



Mouth 



The mouth cavity is divided into an anterior vestibule or labial cavity 

 lying between the lips and the teeth, and a posterior mouth cavity proper 

 or buccal cavity underlaid by the tongue and extending to the posterior 

 margin of the soft palate. The roof of the mouth cavity proper is formed 



