72 PHYSIOLOGY. 



action of the left ventricle, the blood is forced onwards and distri- 

 buted throughout the whole body, with the exception of the lungs, 

 and passes through the capillaries into the veins. 



The pressure of the column of blood against the elastic walls of 

 the arteries, at every contraction of the ventricle, produces what is 

 called the pulse. The sensible pulse of the arteries is synchronous, 

 or nearly so, with the contraction of the ventricle ; it is somewhat 

 later than the heart's beat, especially in the distant vessels, but the 

 difference of time is scarcely perceptible. In the capillaries and 

 veins, the pulse is no longer perceived. 



There is found to be a very close correspondence between the 

 areas of the branches and that of the trunk from which they spring. 

 According to a well-known geometrical law, the areas of circles are 

 as the squares of their diameters. Now, if we add together the 

 squares of the diameters of the branches of a given vessel, we will 

 find that their sum is about equal to the square of the diameter of 

 the parent trunk, showing that the conjoint size of the branches is not 

 greater than that of the main trunk, and that the vascular system 

 cannot be compared to a cone whose apex is at the heart, and whose 

 base is at the circumference of the body, with a regularly increas- 

 ing surface ; but rather to a cylinder, whose diameter is equal through- 

 out, and the pressure upon whose walls is at every point the same. 



The arteries are possessed of three coats, an external, cellular ; 

 a middle, composed of muscular fibres and yellow fibrous tissue ; 

 and an internal, which is serous. Upon the elasticity of the yellow 

 fibrous tissue is dependent the property by which the interrupted 

 force of the heart is made equable and continuous, and which is seen 

 most in the large vessels connected with that organ. The contrac- 

 tility of the muscular fibres, which they have been shown to pos- 

 sess, is concerned in regulating the flow of blood towards particular 

 organs. Their contractility is most plainly seen at a distance from 

 the heart, where the impelling power of this organ becomes almost 

 null. Under these circumstances, the muscular coat becomes an 

 important adjuvant. 



The muscular coat has also another function, that of re^w/a^zw^ 

 the diameters of the tubes in accordance with the quantity of blood 

 to be conducted through them to any part; this is seen in the 

 enlargement of the uterine and mammary arteries, during preg- 

 nancy and lactation, and in their return to their normal size afier 

 these periods are over. These changes are due to the contractility 

 of the muscular fibres of the middle coat, probably regulated by the 

 sympathetic nerve, which is minutely distributed upon the vessels. 

 In the permanent dilatation of arteries, however, in parts that are 

 undergoing enlargement, their nutrition is also increased, the walls 

 being thickened as well as extended. 



In addition to the elasticity and contractility already described, 



