THE VASCULAR SYSTEM. 



221 



tions, the endothelium of the capillaries is seen to form a continuous 

 layer, the cells of which are, as a rule, greatly flattened and present 

 very irregular outlines. 



It is a well-known fact that a migration of the leucocytes occurs 

 from the capillaries and smaller vessels (compare p. 193). In this 

 connection arises the question as to whether or not the cells pass 

 through certain preformed openings in the endothelium of these 

 vessels, the so-cailed stomata, or through the stigmata and intercel- 

 lular cement uniting the endothelial cells. The latter seems more 

 probable, as stomata do not occur normally in the capillary wall. 

 This subject will be further touched upon in the description of the 

 lymphatic system. 



The capillaries connect the arterial and venous precapillary ves- 

 sels, and in general accommodate themselves to the shape of the 

 elements of tissues or organs in which they are situated. In the 



Fig. 176. Endothelial cells of capillary (a) and precapillary (b) from the mesentery of 

 rabbit ; stained in silver nitrate. 



muscles and nerves, etc., they form a network with oblong meshes, 

 while in structures having a considerable surface, such as the pul- 

 monary alveoli, the meshes are more inclined to be round or oval ; 

 such small evaginations of tissue as the papillae of the skin contain 

 capillaries arranged in the shape of loops. In certain organs as, for 

 instance, in the lobules of the liver the capillaries form a distinct 

 netw r ork with small meshes. 



Sinusoids. In connection with the description of capillaries we 

 may here insert a brief account of another type of terminal or 

 peripheral blood-channels, described by Minot under the name of 

 sinusoids; his account is here followed. The sinusoids are also 

 composed only of endothelial cells. They differ, however, from 

 blood-capillaries in shape and size, in their relation to the cellular 

 elements of the tissues in which they are found, and in their devel- 

 opment. They are of relatively large size, and vary between wide 

 extremes. They are of very irregular shape and anastomose freely. 

 "A sinusoid has its endothelium closely fitted against the paren- 



