CHAPTER V. 
THE DIGESTIVE SYSTEM. 
The abdominal cavity or belly is of large size, the nature of 
the food requiring voluminous intestines. The cavity is bounded 
in front by the diaphragm or midriff, which separates it from the 
chest cavity ; posteriorly by the pelvis ; above by the back- 
bone ; laterally by the ribs and their muscles, and below by the 
abdominal muscles. The cavity contains a portion of the gullet, the 
stomach, intestines, liver, spleen, pancreas, kidneys, and the uro- 
genital organs. On opening the cavity shortly after death, it will 
be observed that the intestines are alternately contracting and 
relaxing, but always working down the gut, so as to force onward 
the contents. The movement, like that of the heart, is automatic. 
It will be noted that the cavity is lined by a glistening serous 
membrane which is also reflected over the organs. This membrane 
is known as the peritoneum. Its function is to secrete a small 
quantity of fluid to prevent friction and so permit the continual 
motion of the bowels to go on without injury. 
A striking point is the slight amount or entire absence of fat in 
these creatures. 
The alimentary tract. — This consists of the mouthy through 
which food and water gain entrance to the system, the former being 
ground into a pulp by the teeth and softened by the saliva ; the 
gullet^ which conveys the same by the act of swallowing to the 
stomach, which is the receptacle for it and in which the major part 
of digestion is carried on. By this act the food comes into contact 
with juices secreted by various organs, and these juices have the 
power of altering the food and abstracting from it certain constitu- 
ents which are required to replenish the wear and tear of the 
system. When this act has been sufficiently effected, the altered 
food passes into the intestine, a tube many feet in length, in which 
digestion is completed, and the altered food passed onwards, acquir- 
ing more and more the character of faeces or dung, till it finally 
reaches the rectum, in which the faeces lodge till a certain 
quantity is collected and is then discharged through the amis. 
The mouth. — The cavity of the mouth is remarkably small, a 
fact which is demonstrated by the precision with which the food is 
passed into it by the proboscis, and the small part which the lips 
