ELASTICITY OF THE ARTERIES. 67 



In the smallest arteries, the external coat is thin and disappears just before the ves- 

 sels empty into the capillary system ; so that the very smallest arterioles have only the 

 inner coat and a layer of muscular fibres. Although the greatest part of the muscular 

 fibres in the middle coat of the arteries are arranged at right angles to the course of the 

 vessels, nearly all of the arteries, in the human subject, are provided with longitudinal 

 and oblique muscular fasciculi, which are sometimes external, sometimes internal, and 

 sometimes on both sides of the circular layers. 



The middle coat is composed of circular muscular fibres, without any admixture of 

 elastic elements. In vessels T |-^ of an inch in diameter, we have two or three layers of 

 fibres ; but, as we near the capillaries and as the vessels lose the external fibrous coat, 

 these fibres exist in a single layer. 



The internal coat presents no essential difference from the coat in other vessels, with 

 the exception that the epithelium is less distinctly marked. 



A tolerably-rich plexus of vessels is found in the external coats of the arteries. 

 These are called the vasa vasorum and come from the adjacent arterioles, having no di- 

 rect connection with the vessel on which they are distributed. A few vessels penetrate 

 the external layers of the middle coat, but none are ever found in the internal coat. 



Nervous filaments, principally from the sympathetic system, accompany the arteries, 

 in all probability to their remotest ramifications. These are not distributed in the walls 

 of the large vessels, but rather follow them in their course, their filaments of distribu- 

 tion being found in those vessels in which the muscular element of the middle coat pre- 

 dominates. When we come to treat of the physiology of the organic system of nerves, 

 we shall see that the " vaso-motor " nerves play an important part in regulating the 

 function of nutrition. Lymphatics have not been found in the coats of any of the blood- 

 vessels. 



Course of the Blood in the Arteries. At every pulsation of the heart, all the blood 

 contained in the ventricles, excepting, perhaps, a few drops, is forced into the great vessels. 

 We have already studied the valvular arrangement by which the blood, once forced into 

 these vessels, is prevented from returning into the ventricles during the diastole. The 

 sketch we have given of the anatomy of the arteries has prepared us for a complexity 

 of phenomena in the circulation in these vessels, which would not obtain if they were 

 simple, inelastic tubes. In this case, the intermittent force of the heart would be felt 

 equally in all the vessels, and the arterial circulation would be subject to no modifications 

 which did not come from the action of the central organ. As it is, the blood is received 

 from the heart into vessels endowed, not only with great elasticity, but with contractility. 

 The elasticity, which is the prominent property of the largest arteries, moderates the 

 intermittency of the heart's action, providing a continuous supply to the parts; while 

 the contractility of the smallest arteries is capable of increasing or diminishing the supply 

 in any part, as may be required in the various functions. 



Elasticity of the Arteries. This property, which is particularly marked in large 

 vessels, has long been recognized. If, for example, we forcibly distend the aorta with 

 water, it may be dilated to more than double its ordinary capacity and will resunu its 

 original size and form as soon as the pressure is removed. This simple experiment teaches 

 us that, if the force of the heart be sufficient to distend the great vessels, their elasticity 

 during the intervals of its action must be continually forcing the blood toward the 

 periphery. The fact that the arteries are distended at each systole is abundantly proven 

 by actual experiment ; although the immense capacity of the arterial system, as compared 

 with the small charge of blood which enters at each pulsation, renders the actual dis- 

 tention of the vessels less than we should be led to expect from the force of the heart's 

 contraction. The most satisfactory experiments on this subject are those of Poiseuille. 

 This observer illustrated the dilatation of the arteries in the following way: Having 



