THE DIGESTIVE SYSTEM 



217 



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 lie near 

 the main line of human ancestry, 

 pharynx, esophagus, and stomach 

 are first differentiated in urochor- 

 dates. A liver arises in the cepha- 

 lochordates. The cyclostomes 

 contribute 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. To increase surface for ab- 

 sorbing digested food they develop 

 a spiral fold or "valve" in the 

 intestine. (Fig. 203) They de- 

 velop also a new cavity, the cloaca, 

 to receive the wastes and secretions 

 of the urogenital and digestive 

 systems. 



The amphibians fasten their 

 teeth in a groove in the jaw bone, 



VFJiMIFORM 

 PROCESS 



Fig. 204. — Diagram of the alimentary canal. 

 (From Morris' "Human Anatomy.") 



invent salivary glands, utilize hypobranchial 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. 



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 



