PHYSIOLOGICAL ANATOMY OF THE ARTERIES. 61 



the body, is given off from the left ventricle. Just at its origin, behind the 

 semilunar valves, the aorta has three sacculated pouches, called the sinuses of 

 Valsalva. Beyond this point the vessels are cylindrical. The arteries then 

 branch, divide and subdivide, until they are reduced to microscopic size. 

 The branches, with the exception of the intercostal arteries, which make 

 nearly a right angle with the thoracic aorta, are given off at an acute angle. 

 As a rule, the arteries are nearly straight, taking the shortest course to the 

 parts which they supply with blood ; and while the branches progressively 

 diminish in size, but few are given off between the great trunk and small ves- 

 sels which empty into the capillary system. So long as a vessel gives off no 

 branches, its caliber does not progressively diminish ; as the common carotids, 

 which are as large at their bifurcation as they are at their origin. There are 

 one or two instances in which vessels, although giving off many branches in 

 their course, do not diminish in size for some distance ; as the aorta, which is 

 as large at the point of division into the iliacs as it is in the chest, and the 

 vertebral arteries, which do not diminish in caliber until they enter the fora- 

 men magnum. It has long been remarked that the combined caliber of the 

 branches of an arterial trunk is greater than that of the main vessel ; so that 

 the arterial system, as it branches, increases in capacity. A single exception 

 to this rule is in the instance of the common iliacs, the combined caliber of 

 which is less than the caliber of the abdominal aorta. 



The arrangement of the arteries is such that the requisite supply of blood 

 is sent to all parts of the economy by the shortest course and with the least 

 possible expenditure of force by the heart. Generally the vessels are so situ- 

 ated as not to be exposed to pressure and consequent interruption of the 

 current of blood ; but in certain situations, as about some of the joints, there 

 is necessarily some liability to occasional compression. In certain situations, 

 also, as in the vessels going to the brain, particularly in some of the inferior 

 animals, it is necessary to moderate the force of the blood-current, on account 

 of the delicate structure of the organs in which they are distributed. Here 

 there is a provision in the shape of anastomoses, by which, on the one hand, 

 compression of a vessel simply diverts, and does not arrest the current of 

 blood, and on the other hand, the current is rendered more equable and the 

 force of the heart is moderated. 



The arteries are provided with fibrous sheaths, of greater or less strength, 

 as the vessels are situated in parts more or less exposed to disturbing influ- 

 ences or accidents. 



The arteries have three well defined coats. As these vary very consider- 

 ably in arteries of different sizes, it will be convenient, in their description, 

 to divide the vessels into three classes : 



1. The largest arteries ; in which are included all that are larger than the 

 carotids and common iliacs. 



2. The arteries of medium size ; that is, between the carotids and iliacs 

 and the smallest. 



3. The smallest arteries ; or those less than ^ to ^ of an inch (1-7 to 

 2-1 mm.) in diameter. 



