THE ARTERIES. 89- 



striated muscle found in the large vessels communicating with the heart 

 resembles that of the cardiac wall, with which it is continuous. 



Connective tissue is represented in the arteries and veins by both 

 fibrous and elastic tissue. The former is present usually as bundles of white 

 fibres between the other components of the vessel-wall. The elastic 

 tissue is conspicuous in all arteries, save the smallest, and in many veins. 

 It presents all variations in amount from loose networks of delicate fibres 

 in the smaller vessels to robust plates and membranes in the largest arteries. 

 Within the intima of the latter, the elastic tissue often occurs as sheets 

 broken by pits and perforations, which are, therefore, known as fenestrated 

 membranes. 



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

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

 composing the tubes. These vasa vasorum, as they are called, are usually 

 branches from some neighboring artery ; their favorite situation is the external 

 coat, within which they break up into capillaries that, in the larger vessels, 

 invade the media, but never the intima. The blood from the vessel-wall is 

 collected by small veins that accompany the nutrient arteries, or, as in the case 

 of the veins, empty directly into the venous trunk in whose walls they course. 



The lymphatics are represented by networks of surrounding canals 

 within the adventitia. In certain situations, conspicuously in the brain and 

 the retina, the blood-vessels are enclosed by lymph-channels, \heperivascular~ 

 lymph- sheaths, that occupy the outer coat. 



The nerves distributed to the walls of blood-vessels, except those of the 

 nervous substance of the brain and spinal cord in which nerves are wanting, 

 are numerous and include both efferent and afferent fibres. These form a 

 perivascular plexus around the vessel from which motor (sympathetic) fibres 

 pass to the involuntary muscle, while the sensory fibres end within the outer 

 and inner tunics. Special nerve-endings have been described in both the ex- 

 ternal and internal tunics. 



The Arteries. In cross-sections of arteries of medium size, after the 

 usual methods of preservation which cause some contraction of the vessels, the 

 intima presents a plicated contour, since it follows the foldings of the internal 

 elastic membrane. The latter appears as a conspicuous corrugated light band, 

 marking the outer boundary of the inner tunic. The lining endothelial cells 

 are so thin, that in profile their presence is indicated chiefly by the slightly 

 projecting nuclei. The endothelium and the elastic membrane are separated 

 by a thin layer of fibro-elastic tissue. The media, thick and conspicuous, 

 consists of circularly disposed flat bundles of involuntary muscle separated 

 by plates of elastic tissue. After the usual stainings, these plates appear 

 light and almost uncolored, but after selective dyes, as orcein, the elastica is 

 very conspicuous ( Fig. 1 26) . Delicate bundles of fibrous tissue lie among the 

 musculo-elastic strands. The outer boundary of the media is marked by a 

 more or less distinct external elastic membrane. The adventitia varies in 

 thickness, being relatively better developed in the medium sized arteries than 

 in the larger ones. It consists of bundles of fibrous tissue intermingled with 

 elastic fibres of varying thickness. Sometimes scattered bundles of longi- 

 tudinal muscle are present. The adventitia contains the nutrient blood-ves- 

 sels and the chief lymph-channels and blends with the surrounding areolar 

 tissue without sharp demarcation. 



Followed towards the capillaries, the coats of the artery gradually 

 diminish in thickness. The elastic tissue becomes progressively reduced 

 until it entirely disappears from the middle coat, which then is a purely 



