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HAND-BOOK OF PHYSIOLOGY. 



blood which we see in the capillaries and veins. In the production of a 

 continuous stream of blood in the smaller arteries and capillaries, the 

 resistance which is offered to the blood-stream in these vessels (p. 158), 

 is a necessary agent. Were there no greater obstacle to the escape of 

 blood from the larger arteries than exists to its entrance into them from 

 the heart, the stream would be intermittent, notwithstanding the elas- 

 ticity of the walls of the arteries. 



(c.) By means of the elastic tissue in their walls (and of the muscular 

 tissue also), the arteries are enabled to dilate and contract readily in cor- 

 respondence with any temporary increase or diminution of the total 

 quantity of blood in the body; and within a certain range of diminution 



FIG. 122. 



FIG. 123. 



FIG. 122. Ramification of nerves and termination in the muscular coat of a small artery of the 

 frog. (Arnold.) 



FIG. 123. Transverse section through a large branch of the inferior mesenteric artery of a pig. 

 e, enclothelial membrane; i, tunica elastica interna, no subendothelial layer is seen; m, muscular tu- 

 nica media, containing only a few wavy elastic fibres; ee, tunica elastica externa, dividing the media 

 from the connective tissue adventitia, a. (Klein and Noble Smith.) x 350. 



of the quantity, still to exercise due pressure on their contents; (d.) The 

 elastic tissue assists in restoring the normal state after diminution of its 

 calibre, whether this has been caused by a contraction of the muscular 

 coat, or the temporary application of a compressing force from without. 

 This action is well shown in arteries <which, having contracted by means 

 of their muscular element, after death, regain their average patency on 

 the cessation of post-mortem rigidity (p. 142). (e.} By means of their 

 elastic coat the arteries are enabled to adapt themselves to the different 

 movements of the several parts of the body. 



Tension of Arteries, The natural state of all arteries, in regard at least 

 to their length, is one of tension they are always more or less stretched, 

 and ever ready to recoil by virtue of their elasticity, whenever the oppos- 



