THE BLOOD-VESSELS. 



95 



to a high degree. The direction of the fibres is by no means 

 exclusively transverse. In the outer portions of the media, 

 fibrillated connective tissue occurs (Schultze, von Ebner). In 

 the adventitia {g), finally, the elastic fibro-reticulum (/) 

 acquires in an inward direction, in large mammalia, a very 

 prodigious development. 



The valves of the vessels consist of connective tissue with 

 elastic intermixtures, and the endothelial covering. 



Vasa vasorum is the name given to the capillary vessels 



which occur in the middle and outer layers of the larger trunks, 



and supply the nutritive materials to the walls of the vessel. 



The vascular nerves terminate at the muscles of the 



media. 



We pass to the arrangement of the 

 capillary vessels in the human body. 



It is known that they do not occur 

 everywhere. Thus, the epithelial struc- 

 tures, with the crystalline lens, the cornea 

 of the eye, and the permanent cartilages 

 are non-vascular. 



A peculiarity of the capillary division 



Fig. 91. — Vascular net- 

 work of a transversely stri- 

 ated muscle ; a, arterial, 

 6, venous vessel; c, d, 

 the capillary net-work. 



Fig. 92. — A pulmonary alveolus of the calf ; 

 a, larger blood-vessels, which run in the pari- 

 etes of the alveoli ; b, capil ary net-work ; c, 

 epithelial cells. 



consists in this, that the tubes by giving off branches do 

 not become narrowed to a noticeable degree, and that by 



