XIII 



PHYLUM CHORDATA 



81 



jaws and agree with them in structure, so that there can be no 

 doubt as to the homology of the two. Teeth are, in fact, to be 

 looked upon as portions of the exoskeleton which have migrated 

 from the skin into the buccal cavity, and even into the pharynx, 

 and have there increased in size and assumed special functions. 



The tongue is a muscular elevation of the floor of the mouth, 

 supported by the basi-hyal, and usually more or less protrusible. 

 The roof of the buccal cavity in the embryo sends off a pouch, the 

 mtuitary diverticulum (Fig. 715, A, pty. s.\ which grows upwards 

 and, losing its connection with the mouth, becomes attached to 

 the ventral surface of the brain as the pituitary body (pty. b.). It 

 may correspond with the sub-neural gland of Urochorda. 



In terrestrial Craniata buccal glands are present, opening by 

 ducts into the mouth : the most important of these are the race- 

 mose salivary glands which secrete a digestive fluid, saliva, capable 

 of converting starch into sugar. There are also two large and 

 highly characteristic digestive glands in the abdominal cavity, 

 both developed as outpushings of the intestine, but differing 

 greatly from one another, in their fully developed state, both in 

 outward appearance and in histological structure : these are the 

 liver and the pancreas. 



The liver is (Fig. 715, A,Zr.)a dark-red organ of relatively immense 

 size : it not only secretes a digestive juice, the bile, which has the 

 function of emulsifying fats, but also forms an amyloid substance 

 called glycogen or 

 animal starch, 

 which, after being 

 stored up in the 

 liver-cells, is re- 

 stored to the blood 

 in the form of 

 sugar. The liver is 

 formed of a mass 

 of polyhedral cells 

 (Fig. 725, I.) with 

 minute intercellu- 

 lar spaces which 

 receive the bile 

 secreted from the 

 cells and from 

 which it passes to 

 the ducts (b). The 

 pancreas (Fig. 7 15, 



A,/>?i.) is a racemose gland, and secretes pancreatic juice, which acts 

 upon proteids, starch, and fats. The ducts of both glands usually 

 open into the anterior end of the intestine : that of the liver (b. d.) 

 generally gives off a blind offshoot ending in a capacious dilatation, 



VOL. IT G 



FIG. 725. Diagram of structure of liver. 6, a small branch of 

 hepatic duct ; b', its ultimate termination in the intercellular 

 spaces; c, blood capillaries; I, liver cells, (trom Huxley's 

 Physiology.) 



