36 THE VASCULAR SYSTEM OF THE FEOG 



truncus into right and left brandies, and note tlu.it though each 

 branch is apparently a single vessel its cavity is really divided 

 into three vessels corresponding to the three aortic arches. Pass 

 bristles down these aortic arches, and note the points at which they 

 severally open into the truncus arteriosus. 



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 most 

 important are the following : 



a. The subcutaneous lymph sacs are the large 



cavities between the skin and the muscles, 

 which have already been seen when remov- 

 ing 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. 19.) The body-cavity itself also 

 communicates with the lymphatic system 

 through small openings or stomata in the 

 peritoneum. 



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

 tractile sacs placed at points Avhere the lymphatic 

 vessels communicate with the veins. They are quite 

 transparent. 



a. The anterior lymph hearts lie immediately 

 behind the transverse processes of the third 



