34 THE VASCULAR SYSTEM OF THE FROG. 



E. The Lymphatic System. 



The lymphatic system forms an accessory part of the vascular 

 system. Its main divisions are as follows : 



1. The lymphatic vessels are a series of thin- walled tubes, 



very variable in diameter and irregular in shape, which 

 traverse all the parts and organs of the body and are 

 in free communication with the veins. They are of 

 small size, and can only be recognised with the 

 microscope. 



2. The lymph sacs are large irregular spaces communi- 



cating with the lymphatic vessels. The rnost 



important are the following : 



a. The suhcutaneous lymph sacs are the large 

 cavities between the skin and the muscles, 

 which have already been seen when removing 

 the skin. They are separated from one 

 another by narrow septa of connective tissue, 

 which bind the skin to the underlying body- 

 wall. 

 "b. The abdominal lymph sacs are the large 

 spaces along the dorsal surface of the body- 

 cavity, ventral to the kidneys, and between 

 the peritoneum and the body-walls. {See 

 Fig. 2, p. 17.) The body cavity itself also 

 commiuiicates with the lymphatic system 

 through small openings or stomata in the 

 peritoneum. 



3. The lymph hearts are two pairs of small globular con- 



tractile sacs placed at points where the lymphatic 



vessels communicate with the veins. 



a. The anterior lymph hearts lie immediately 

 behind the transverse processes of the third 

 vertebra, and beneath the shoulder girdle : 

 they open into the subscapular veins. 

 h. The posterior Ijrmph hearts lie at the sides of 

 the urostyle, close to its hinder end. They 

 communicate by short vessels wdth the 

 femoral veins. Their pulsations can easily 

 be seen in a living frog. 



