PHYSIOLOGICAL ANATOMY OF THE ARTERIES. 241 



which the secretions are formed, and in short are the avenues 

 of supply to every part of the organism. The supply of 

 blood regulates, to a considerable extent, the process of nu- 

 trition, and has an important bearing on the general and 

 special functions. The physiological processes necessarily 

 demand considerable modifications in the quantity of arterial 

 blood which is furnished to parts at different times. For ex- 

 ample, during secretion, the glands require twice or three 

 times as much blood as in the intervals of their action. The 

 force of the heart, we have seen, varies but little within the 

 limits of health, and the conditions necessary to the proper 

 distribution of blood in the economy are regulated almost 

 exclusively by the arterial system. These vessels are not in- 

 ert tubes, but are endowed with elasticity, by which the cir- 

 culation is considerably facilitated, and with contractility, by 

 which the supply to any part may be modified, independent- 

 ly of the action of the heart. Sudden flushes or pallor of the 

 countenance are examples of the facility with which this may 

 be effected. It is evident, therefore, that the properties of 

 the coats of the arteries are of great physiological importance. 

 We will then commence the study of this division of the cir- 

 culatory system with a consideration of its physiological 

 anatomy. 



Physiological Anatomy of the Arteries. 



The vessels which carry the venous blood to the lungs 

 are branches of a great trunk which takes its origin from the 

 right ventricle. They do not differ in structure from the 

 vessels which carry the blood to the general system, except 

 in the fact that their coats are somewhat thinner and more dis- 

 tensible. The aorta, branches and ramifications of which sup- 

 ply all parts of the body, is given off from the left ventricle. 

 Just at the origin, behind the semilunar valves, the aorta has 

 three sacculated pouches, called the sinuses of Yalsalva. Be- 

 yoiid this point the vessels are cylindrical. As we recede 

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