224 FIRST LESSONS IN ZOOLOGY. 



tinues its backward course (^Ao) just in front of the vertebral column 

 into the abdomen. The trachea gives off a bronchus to each lung 

 {Lu). The lungs are sacculated elastic organs, with no main cen- 

 tral cavity. They are separated dorsally by a thin median vertical 

 membrane {M), the mediastinum, the equivalent of the mesentery in 

 the abdomen. Lying on the side of the vertebral column can be 

 seen part of one of the two chains of sympathetic nervous ganglia 



{S). 



The abdominal cavity contains the principal reproductive, excre- 

 tory, and digestive organs. The cesophagus terminates in the stom- 

 ach almost immediately below the diaphragm. The stomach (St) 

 occupies a transverse position, its larger {ca/rdiac) end, which re- 

 ceives the oesophagus, lying on the left, the smaller {pyloric) end 

 on the right. The pylonis is furnished with a sphincter muscle, a 

 ring of fibres, which can completely close the orifice by their contrac- 

 tion. The stomach is followed by the long, coiled small intestine (In). 

 The posterior portion of the intestine is somewhat dilated, and is called 

 the colon, which passes into the wide terminal rectum (R). The whole 

 abdominal portion of the ihtestinal canal is suspended from the me- 

 dian dorsal line by a thin membrane, the mesentery, which forms sev- 

 eral folds, the most striking of which is the omentum or grand epip- 

 loon (Om). This fold, when in situ, hangs down from the stom- 

 ach like an apron, covering over the intestines ventrally. Upon 

 opening the walls of the abdomen, it is the first structure met with. It 

 usually contains a great deal of fat. Its principal function is sup- 

 posed to be to prevent the loss of heat. The omentum is present in 

 all mammals, but is least developed in Cetaceans, being most promi- 

 nent in Carnivora and Ruminants. Connected with the intestine are 

 two glands, the liver (lA) and pancreas (Pan). The liver is large and 

 lies directly underneath the diaphragm. One part of it has been cut 

 across and left in situ to show the oblong gall-bladder (G). The pan- 

 creas is very much elongated, and lies upon the upper part of the intes- 

 tine, the so-called duodenum. The spleen (8p) is closely connected 

 with the stomach, and is of an elongated shape. 



The kidneys (K) are large and oval, and lie on either side of the 

 vertebral column; the aorta passes between them, giving off a renal 

 branch to each gland. A delicate ureter passes from each kidney 

 obliquely across the rectum to the large flask-shaped bladder (B). A 

 urethra, not shown in the figure, arises from the bladder posteriorly 

 and opens immediately below the anus. 



The ovary (On) is small and is placed near the open end of the ovi- 

 duct or Fallopian tube, which can be seen in the figure extending 

 alongside the rectum above the bladder (Minot). 



