674 



HUMAN ANATOMY. 



the endothelium into distinct cells is less evident in the capillaries than in the larger 

 vessels, in some cases a continuous syncytial sheet replacing the definitely outlined 

 plates. The presence of a relatively small oval nucleus is readily demonstrated by 

 suitable stains. 



The involuntary muscle varies in amount, from the imperfect single layer of 

 muscle-cells found in the arterioles, to the robust muscular coat of many lamellae in 

 the larger arteries. It is relatively best developed in arteries of medium size, where 

 the 'muscle occurs in distinct broad or sheet-like bundles between the strands of elastic 

 tissue. The component fibre-cells are short «and often branched and, for the most 

 part, circularly disposed. The distribution of the muscular tissue is much less 

 regular and constant in the veins than in the arteries, since in many it is scanty, 

 in some entirely wanting, and in a few veins excessive, occurring in both circular and 

 longitudinal layers. The striated viuscle found in the large vessels communicating 

 with the heart resembles that of the cardiac wall from which it is derived. 



Connective-tissue is represented in the arteries and veins by both fibrous and 

 elastic tissue. The former is present as delicate or coarser bundles of fibrillae that 

 extend between the other components of the vascular wall. 



Fig. 634. 



B. 



A, endothelium of aiteriole after silver-staining; X 200. 

 more highly magnified. X 500. 



B, endothelial cells 



The elastic tissue is very conspicuous in all arteries save the smallest' and in 

 many veins. It presents all variations in amount and arrangement from loose net- 

 works of delicate fibres in the smaller vessels to robust plates and membranes in the 

 largest arteries. Within the intima of the latter, the elastica often occurs as sheets 

 broken by pits and perforations, which are, therefore, known as foiestrated viem- 

 brayies. 



Nutrient blood-vessels are present within the walls of all the larger vessels, 

 down to those of i mm. in diameter, and provide nourishment for the tissues com- 

 posing the tubes. These vasavasoriiui, as they are called, are usually branches from 

 some neighboring artery, their favorite situation being the external coat within which 

 they ramify, breaking up into capillaries that, in the larger vessels, invade the adja- 

 cent media. The blood from the vascular wall is collected by small veins that accom- 

 pany the nutrient arteries, or, as in the case of the veins, empty directly into the 

 venous trunk in whose walls they course. 



Lymphatics are represented by spaces both within the muscular tissue and 

 beneath the endothelium. In certain situations, conspicuously in the brain and the 

 retina, the blood-vessels are enclosed within lymph-channels, 'Ca^ perivascular lyt7iph- 

 sheaths, that occupy the adventitia. 



The nerves distributed to the walls of blood-vessels, especially to the arteries, 

 are numerous and include both sympathetic and spinal fibres. The former are des- 



