166 BLOOD-VESSELS. 



the Telocity, by increasing the extent of surface over which the blood 

 moves, and consequently the amount of impediment from friction; 

 still it does not satisfactorily appear why such an end should be 

 provided for in the several cases in which arteries are known to follow 

 a tortuous course. The same remark ap])lies to the peculiar arrange- 

 ment of vessels named a " rete mirabile," where an artery suddenly 

 divides into small anastomosing branches, which in many cases unite 

 again to reconstruct and continue the trunk. Of such 7'efia mirahilia 

 there are many examples in the lower animals, but, as already remarked, 

 the purpose Avhich they serve is not apparent. The best known instance 

 is that named the rcfe mirabile of Galen, which is formed by the intra- 

 cranial part of the internal carotid artery of the sheep and several other 

 quadrupeds. 



Physical Properties. — Arteries possess considerable strength and a 

 very high degree of elasticity, being extensible and retractile both in 

 their length and their width. When cut across, they present, although 

 empty, an open orifice ; the veins, on the other hand, collapse, unless 

 when prevented by connection with surrounding rigid parts. 



Structure. — In most parts of the body the arteries are inclosed in 

 a sheath formed of connective tissue, and their outer coat is connected 

 to the sheath by filaments of the same tissue, but so loosely that, when 

 the vessel is cut across, its ends readily shrink some way within the 

 sheath. The sheath may inclose other parts along with the artery, as 

 in the case of that enveloping the carotid artery, which also includes 

 the internal jugular vein and pneumo-gastric nerve. Some arteries 

 want sheaths, as those for example which are situated within the cavity 

 of the cranium. 



Independently of this sheath, arteries (except those of minute size 

 ■whose structure will be afterwards described with that of the capillaries) 



Kg. 107. 



cl fi^ 



zy^ 



Fig. 107. — Transverse Vertical Section of Posterior Tibial Artery (Man). 75 



Diameters. 



a, Eiiithelioid and Subepithelial layers of inner coat ; b, elastic layer of inner coat, 

 appearing as a bright line in section ; c, muscular layer (middle coat) ; d, outer coat, con- 

 sisting of connective tissue bundles, which become more loosely arranged toward the 

 exterior. In the interstices (seen as white spaces) of the bundles arc some connective tissue 

 nuclei, and, especially near the muscular coat, a number of fine elastic fibres cut across. 



