THE BLOOD-VESSELS. 93 



group of layers essentially preserve in their membranous 

 layers the nature of the elastic tissue, and present the most 

 heterogeneous varieties of the same with a longitudinal 

 arrangement. The middlemost group changes into a system 

 of alternating layers of elastic tissue and smooth muscles, 

 both with a transverse direction, or also of connective tissue. 

 The tunica media remains much thinner in veins than in 

 arteries of similar size, and the result is that the walls of the 

 former vessels are thinner. The endothelial cells of the 

 arteries appear as narrow, lancet-shaped lamellae ; those of the 

 veins are shorter and broader (p. 29). 



Taking a small vein of about 0.25 mm. in calibre, we find 

 succeeding the epithelium a serosa with fine, elastic, longi- 

 tudinal reticula. The middle layer consists of several mus- 

 cular layers, which have between them elastic reticula and 

 connective-tissue layers. The adventitia shows longitudinally 

 running connective tissue and a contingent of elastic fibres. 



The appearance is different in middle-sized veins. The 

 serosa has here become a group of layers. We now meet 

 with homogeneous or striped layers with longitudinally 

 arranged spindle cells, elastic membranes or elongated reti- 

 cula. Indeed, even the elements of the smooth muscles may 

 be continued in these inner groups of layers. The middle 

 layers consist of connective tissue running transversely, with 

 elastic reticula arranged in the same manner, and smooth 

 muscles. Nevertheless, isolated elastic layers with longitu- 

 dinal fibres also occur here. The adventitia is as usual ; still 

 it may also harbor contractile fibre cells. 



The largest veins possess a similar serosa, though without 

 the smooth muscles, while the media remains undeveloped 

 and may he entirely absent. It shows scanty muscular 

 elements, permeated by transverse connective tissue. Elastic 

 longitudinal fibro-reticula have likewise maintained their posi- 

 tion here. In the thick adventitia of many veins, one meets 

 toward the interior with thick longitudinal muscles, as, for 

 instance, that of the pregnant uterus, while the sinuses of the 

 dura mater arj entirely without muscles. 



