224 



VERTEBRATE ANIMALS. 



(s). 

 the 



in the mouth to the action of a special fluid — the saliva — -whicli acts 

 chemically as well as mechanically upon the food, and which is 

 secreted by special glands, known as the " salivary glands." From 

 the mouth the food passes through a muscular tube — the gullet, or 

 us (fig. 162, ff) — to the proper digestive cavity, or stomach 

 Here it is subjected to the action of a special digestive fluid — 

 gastric juice " — and is converted into a thick pasty fluid, which 

 is called chyme. From the stomach 

 the chyme passes into a long con- 

 voluted muscular tube, which is called 

 the " small intestine " {sm). Here it 

 is subjected to the action of two other 

 digestive fluids, called the " bile " and 

 " pancreatic juice," as well as to the 

 fluids secreted by the intestine itself. 

 The bile is secreted by a large gland, 

 which is known as the "liver," whilst 

 the pancreatic juice is produced by an- 

 other, termed the " pancreas," both 

 pouring their secretion into the upper 

 part of the small intestine. By the 

 combined action of these digestive 

 fluids the chyme is iiltimately con- 

 verted into a milky fluid, which is 

 called chyle, when it is fit to be taken 

 up into the blood-vessels. The small 

 intestine finally opens into a tube of 

 larger diameter, which is called the 

 "large intestine" {hn), and this opens 

 on the surface of the body by an anal 

 aperture. In the large intestine the 

 last remaining portions of the food 

 which can be rendered useful are ab- 

 sorbed into the blood, the indigestible 

 portions being ultimately got rid of as 

 useless. The fluid products of diges- 

 tion {chyle) are chiefly absorbed from 

 the intestinal canal by a set of special vessels, which are present in all 

 Vertebrates, and which ai'e called the lueteuls, from the milky fluid 

 which they contain. These lacteals combine to form a large trunk, 

 by which tlieir contents are ultimately added to the circulating 

 blood. Part of the products of digestion is absorbed by the veins 

 which ramify on the intestinal canal, and which ultimately unite to 

 form a great vessel, called the "vena port;i'," which goes to the liver. 



Fig. Iij2. — Digestive system of a. 

 Mainiiial. g Gullet or n-sopliagus : 

 s Stniimch ; sm Small intestine ; 

 Im Large intestine ; r Large in- 

 testine terminating in its Jinal 

 liortion, called the "rectum." 



