66 AN INTRODUCTION TO ZOOLOGY 



dorsally to the acetabulum into the coelom. Here it divides into two 

 branches, the external iliac vein going forward approximately 

 parallel with the ilium, and the pelvic vein running ventro-mesially 

 across the hinder part of the pelvis. The sciatic, draining the muscles 

 and skin of the back of the thigh, runs up to join the external iliac 

 a short distance behind the kidney. The trunk formed by this 

 union is the renal portal vein, and it passes to the lateral edge of the 

 kidney, along which it runs, gradually decreasing in calibre. Near 

 the middle of its course along the kidney the dorso-lumbar vein 

 opens into it, bringing blood from the back and lumbar region, and 

 from it pass off a number of branches (venae renales advehentes) 

 into the kidney substance. 



The two pelvics unite in the middle line on the inner surface 

 of the body wall to form the single anterior abdominal vein, which 

 in its forward course underlies the linea alba. Shortly after its 

 origin it receives a well-marked tributary from the bladder, the 

 vesicular vein, and a number of smaller parietal veins enter it on its 

 way forward. At the level of the posterior end of the heart it 

 passes up to the liver, receiving a small vessel, the cardiac vein, as 

 it does so. It enters the hepatic portal vein as this vessel divides 

 into two, and from just before its point of union it sends a small 

 branch to the left lobe of the liver. 



The hepatic portal vein is formed by a number of factors from 

 the various parts of the alimentary canal, chief among which 

 are the intestinal, composed of branches from the intestine and 

 duodenum, the splenic from the spleen and the gastric from the 

 stomach. It divides into two, one going to each lobe of the liver, and 

 it receives the anterior abdominal just where it divides. 



Lymphatic System. 



While traversing the capillaries a certain amount of liquid 

 passes off from the blood so that the tissues are bathed in this fluid, 

 called the lymph. In addition to the arteries, veins and capillaries 

 already described, there is present in all parts of the body a network 

 of small vessels for the conveyance of the lymph, the lymphatics, 

 which have extremely thin walls. They collapse very readily, and 

 as they only contain the colourless lymph they are generally over- 

 looked in dissection. Connected with the lymphatics are large 

 spaces also containing lymph. It will be remembered that the skin 

 of the frog is only loosely attached by means of septa to the under- 

 lying muscles of the body. The spaces separated from one another 

 by these partitions form the. great series pf sub-cutaneous lymph 

 sacs. Beneath the peritoneum and the muscles of the body wall 

 are also lymph spaces, which communicate with the ccelom by 



