MOVEMENT OF THE BLOOD IN THE ARTERIES. 



323 



3. Movement of the Blood in the Arteries. 



250. The Blood propelled from the Heart is distributed to the body in 

 general by a system of Arteries, which may be likened in its arrangement 

 to the trunk and branches of a tree, except that very frequent communica- 

 tions or anastomoses exist among these branches, so that, by continual sub- 

 division and inosculation, their distribution comes more and more to resem- 

 ble the capillary network in which they terminate (Fig. 129). Although 



FIG. 129. 



Web of Frog's foot, stretchiug between two toes, magnified three diameters : showing the bloodves- 

 sels, and their anastomoses : 1, 1, veins ; 2, 2, 2, arteries. 



the diameters of the branches, at each subdivision, together exceed that of 

 the trunk, yet there is but little difference in their respective areas. What 

 difference does exist, however, is with few exceptions, in favor of the branches; 

 and thus it happens that there is a gradual increase in the capacity of the 

 arterial system from its centre towards the capillaries, whose capacity is many 

 times greater. The Arteries exert a most important influence upon the 

 movement of blood through them, in virtue of the physical and vital prop- 

 erties of their walls, or rather of their middle or fibrous coat, which alone 

 is possessed of contractile properties. We find in this coat a layer of yellow 

 Elastic tissue, which is much thicker in the larger arteries, in proportion to 

 their size, than in the smaller. On the inside of this is a layer of annular 

 fibres composed of Muscular fibre-cells, mingled with areolar tissue; 1 the 

 muscular element being much more abundant in the smaller arteries, than 

 in the larger. To the former tissue is due the simple elasticity ' 2 of the arte- 



1 See Prof. Kolliker's Manual of Human Histology (Sydenham Society's edit.), 

 vol. ii, p. 291, and Handbuch der Gewebelehre, 5 Auflag, p. 583. Eberth in Strieker's 

 Human and Comp. Hist., Syd. Soc. Trans., 1870, vol. i, Art. Bloodvessels. 



In some vessels, as in the splenic and umbilical, and in the dorsal artery of the 

 penis, a few longitudinal muscular fibres are found in the external coat, whilst in 

 others, as the renal, hepatic, splenic and crural, longitudinal fibres are found in the 

 internal coat. (Kemak, Eberth.) 



2 Wiindt (Physiologie, 1813, p. 287) finds the coefficient of elasticity of the arte- 

 ries ({. e., the weight necessary to be applied to double the length of a given piece), 

 to be 1120 grains, and that of the veins 1465 grains. In both sets of vessels the elas- 



