34 ELEMENTARY PHYSIOLOGY less. 



of course, narrow the calibre of the vessel, just as squeezing 

 it with tlie hand or in any other way would do ; and this 

 contraction may go so far as, in some cases, to reduce the 

 cavity of the vessel almost to nothing, and to render it 

 practically impervious. 



The state of contraction of those muscles of the small 

 arteries is regulated, like tliat Of other muscles, by 

 their nerves ; or, in other words, the nerves supplied 

 to the vessels determine wlietlier the passage through 

 these tubes should be wide and free, or narrow and ob- 

 structed. Thus while the small arteries lose the function, 

 which the capillaries possess, of directly irrigatnig the 

 tissues by transudation, they gain that of regulating 

 the supply of fluid to the irrigators or capillaries 

 themselves. The contraction, or dilation, of the arteries 

 which supply a set of capillai'ies, comes to the same result 

 as lowering or raising the sluice-gates of a system of 

 irrigation -canals. Thus the one great and all-important 

 use of the mu.scular tissue of the smaller arteries is 

 to deteniiine and cantrol the supply of blood to each part of 

 the body, according to the varying needs of that j)art. 



The smaller arteries and veins severally unite into, 

 or are branches of, larger arterial or venous trunks, 

 which again spring from or unite into still larger ones, and 

 these, at length, communicate by a few princi^jal arterial 

 and venous trunks with the heart. 



(ii) The Structure of a Vein. — The wall of a vein 

 is structurally similar to that of an artery in so far that it 

 consists essentially of the same three layers or coats, 

 but the distinction between the middle and outer coats, 

 so easily made out in an artery, is usually very obscure 

 in a vein or even altogether wanting in some veins. 

 It differs from that of an artery chiefly in the fact that it 

 is thinner, less muscular and less elastic, and contains 

 relatively more connective tissue. Hence the walls of a 

 vein collapse or fall together when the vessel is empty, 

 whereas those of an artery do not. 



