142 ANATOMY AND PHYSIOLOGY. 



performed ; the surface is easily chilled, and the diseases have fre- 

 quently a low insidious character. When the complexion is fair, 

 this has been called the phlegmatic temperament, and the melan- 

 cholic when the complexion is dark. With a feeble circulation, the 

 general health never can be good ; and hence we find the action of 

 the heart weak in most delicate persons. 



The arteries, like the intestines, are composed of three coats, and 

 the middle one is generally considered to be muscular, in order to 

 assist the contractions of the heart ; but its muscularity is by no 

 means so marked as is the muscularity of the intestinal canal. 

 These coats possess also different degrees of distensibility, the 

 inner one having the least. This gives rise to the remarkable cir- 

 cumstance, that when, as in those horrid accidents that are some- 

 times caused by machinery, a limb is torn off, frequently not a 

 spoonful of blood will be lost. The reason is, that the inner coat, 

 which is ruptured first, curls up, and, assisted by the outer coat, 

 forms a plug in the blood-vessels. These coats are sometimes dis- 

 tended more gradually by the continued impulse of the heart, 

 constituting the disease called aneurism. The sac thus formed, if 

 on the largest vessels, occasionally attains the size of a child's 

 head, and produces instantaneous death when it ultimately bursts. 



The part of the circulating system most liable to disease is the 

 valves, and especially those of the left side of the heart. Ossifi- 

 cation, or the disposition of bone in their substance, is what most 

 commonly affects them. As might be anticipated, the blood regur- 

 gitates and stagnates, and great distress in breathing, dropsy, &c., 

 are the consequences. The only other valves in the circulating 

 system are in the veins, and they seldom become diseased. 



The cavities of the left side of the heart contain arterial blood, 

 and those of the right side, venous blood. 



The vessels which convey arterial blood into all the organs take 

 their origin from the left ventricle of the heart, through the medium 

 of a single trunk, called the aorta. 



This great artery first mounts upwards towards the base of the 

 neck, then bends downwards, forming a sort of crook, passes be- 

 hind the heart and descends vertically, in front of the spine, to the 

 lower part of the belly. In its course, the aorta gives off a great 

 number of branches, the principal of which are : 



1st. The two carotid arteries mount along the sides of the neck, 

 and supply the head with blood. 



2d. The two arteries of the upper extremities, successively 



