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TEXT-BOOK OF PHYSIOLOGY. 



tively by raising the intra-arterial pressure from o to 200 mm.- of 

 mercury. 



The contractility permits of a variation in the amount of blood 

 passing into a given area in a unit of time. Normally each artery 

 has a certain average caliber due to a given contraction of the mus- 

 cular coat. Beyond this average condition the artery can pass in 

 one direction or the other by either a relaxation or increased con- 

 traction of the muscular coat. During the functional activity of 

 any organ or tissue there is need for an increase in the amount of 

 blood beyond that supplied during inactivity or rest. This is ac- 

 complished by a relaxation of the muscle-fibers, an increase in diam- 

 eter, and an increase in the outflow of blood. With the cessation of 



activity the muscle- 

 fibers contract, the 

 diameter diminishes, 

 ' and the outflow of 

 blood is reduced in 

 amount. An increased 

 contraction of the 

 muscle-fibers beyond 

 the average dimin- 

 FIG. 135. TRANSVERSE SECTION OF PART OF THE 



WALL OF THE POSTERIOR TIBIAL ARTERY (MAN). 

 (Scha/er.) a. Endothelium lining the vessel, 

 appearing thicker than natural from the contrac- 

 tion of the outer coats, b. The elastic layer of 

 the intima. c. Middle coat composed of muscle- 

 fibers and elastic tissue, d. Outer coat consisting 

 chiefly of white fibrous tissue. (From Yeo's "Phys- 

 iology. ") 



ishes the outflow of 

 blood, and if suffi- 

 ciently great may give 

 rise to anemia and 

 pallor. The contrac- 

 tile elements at the 

 periphery of the arte- 

 rial system, in the so- 

 called artenole region, regulate the supply of blood to the tissues in 

 accordance with their functional needs. 



Structure of the Capillaries. The capillaries are small vessels 

 that connect the arteries with the veins. Though different in struc- 

 ture from a small artery or vein, there is no sharp boundary between 

 them, as their structures pass imperceptibly one into the other. A 

 true capillary, however, is of uniform size in any given tissue and does 

 not undergo any noticeable decrease in size from repeated branchings. 

 The diameter varies in different tissues from 0.0045 mm. to 0.007 S 

 im, just sufficiently large to permit the easy, passage of a single 

 red corpuscle. The length varies from 0.5 mm. to i mm. The 

 wall of the capillary (Fig. 136) is composed of a single layer of nu- 



mat rf a i en Tf ^ ^ ^ ****** <*** united *>y a cement 

 S an U mh Ug f f Xtrem , dy Sh rt ' the ca P illarie * divide and sub- 

 a number of times, forming meshes or networks, the closeness 

 and general arrangements of which vary in different localities 



