THE LYMPHATIC SYSTEM. 



173 



by elastic fibres and connective-tissue fasciculi. In the larger 

 trunks, smooth muscular elements form a middle layer. The 

 endothelial cells of the lymphatics have a more sinuous outline 

 than the spindle-shaped cells of the blood-capillaries ; they are 

 often irregularly dentated like the cranial sutures. The calibre 

 of the lymphatics is also much more irregular than that of 

 blood-vessels. As they increase in size, the tissues external to 

 the endothelium assume more and more the character of ves- 



Fio. 78. Central tendon of the rabbit, treated with solution of nitrate of silver, the most superficial 

 serous layer immediately adjoining the pericardium being shown : a, lymphatic capillaries ; ft, their ori- 

 gin ; c, serous canals with communications ; d, serous canals equal in width to the origin of the lympha- 

 tic vessels ; e t blood-vessel with epithelial cells. Magnified 300 diameters. Recklinghausen. 



sels with independent walls ; they finally resemble the veins 

 in the largest trunks, except that they possess more muscular 

 tissue than the latter. 



The diameter of the lymphatic capillaries is very variable ; 

 they are generally larger than the blood-capillaries, ranging 

 from 0.0130.045 mm. (Frey). Branches of 0.2256 .2609 mm. 

 may possess three layers (Kolliker). The vessels are richly 

 supplied with valves, which are formed from the intima. 



Variations in shape. Here let us consider an important 

 characteristic of the lymphatic system, viz., its irregularity. 



