146 MORPHOLOGICAL TYPES 



connected with the stomach by the lesser omentum (mesen- 

 tery), and with the floor of the peritoneal cavity by a small 

 ventral mesentery, the falciform ligament. The gall-bladder 

 is green in colour. The pancreas lies in the mesentery which 

 stretches between the two arms of a loop formed by the 

 duodenum. 



The small intestine is lined with countless finger-shaped 

 processes called villi which absorb the products of digestion. 

 Along the wall of the intestine are masses of lymphatic tissue 

 known as Peyer's patches, from which lymphocytes pass into 

 the cavity of the intestine. In the rabbit the small intestine 

 is over two yards long, and it ends in a chamber called the 

 sacculus rotundus, with which the ccecum and the large 

 intestine connect. The ccecum or blind gut ends blindly as 

 its name implies, and at its extremity is the vermiform appendix 

 which contains much lymphatic tissue. The ccecum is a 

 structure commonly found in herbivorous animals, for in it 

 cellulose is digested with the help of bacteria. It is usual to 

 find it reduced or absent in carnivorous forms. The large 

 intestine or colon connects with the caecum near the opening 

 of the sacculus rotundus, and leads to the rectum and anus. 



Respiratory System.— Most of the structures concerned with 

 respiration have already been described in connexion with 

 the mouth and the pleural cavities. The larynx is protected 

 by a number of cartilages (thyroid, cricoid, and arytenoid) to 

 which muscles are attached. Internally, it contains the vocal 

 cords. The trachea which is kept open by cartilaginous rings, 

 leads from the larynx to the point where the two bronchi arise. 

 Each bronchus leads to a lung, and becomes subdivided into 

 larger and larger numbers of increasingly smaller air-spaces. 

 The mammalian lung is not a vascular hollow sac such as the 

 lung of the newt or the lizard ; its cavity is repeatedly sub- 

 divided so that it appears to be filled with spongy tissue in 

 which blood-capillaries circulate, surrounded on all sides by 

 the minute air-spaces. The surface of contact between air- 

 spaces and blood-vessels is very great ; in man, for example, it 

 is about thirty times the area of the body-surface. 



Vascular System.- — The heart contains four chambers, two 



