THE RAT 473 



External features. In addition to obvious differences of shape 

 and form, the big toe is present ; there are usually six pairs of 

 teats, three pectoral and three abdominal. 



Alimentary system. The tonsils are absent, or perhaps are 

 represented by scattered pits in the throat. The dental formula is 



-. There is no infraorbital salivary gland, but there is a 



1003 



major sublingual lying on the antero-lateral surface of the sub- 

 maxillary gland, and a minor sublingual beneath the mucous 

 membrane on the floor of the mouth. The pancreas is more 

 diffuse than in the rabbit, and extends into the omentum which 

 joins the spleen to the stomach. There is no gall bladder. The 

 caecum differs considerably from that of the rabbit ; it is relatively 

 shorter, it does not show the well-marked three limbs, and it is 

 not spirally constricted. It is divided about the middle by a 

 slight groove, which marks off the distal part as the appendix. 



Circulatory system. The coeliac artery divides into three, the 

 gastric, splenic and hepatic. The iliolumbar arteries usually arise 

 from the dorsal aorta ; the posterior mesenteric artery arises from 

 a point near the origin of the iliac arteries. The internal and 

 external jugular veins open separately into the anterior vena 

 cava ; there is a small anastomosis between the internal jugular 

 veins but apparently none between the external veins. The 

 azygos vein is on the left side and enters the left anterior vena 

 cava. There is one pulmonary vein from each lobe of the lung, 

 that is, one on the left, four on the right. There is a single hepatic 

 vein. The external iliac vein and the elements of the internal 

 iliac or hypogastric usually join to form a common iliac. 



Urinogenital system. Near its opening each vas deferens gives 

 off a large seminal vesicle or vesicular gland. There is no evidence 

 that it stores sperms, but its secretion adds much bulk to the 

 semen, and it secretes the fructose on which the sperms feed. 



Nervous system. The brachial plexus is formed from the first 

 thoracic and last four cervical nerves, with usually a small branch 

 from the second thoracic. There is a lumbosacral plexus formed 

 from all six lumbar nerves and the first sacral. 



M.z. — 16 



