CIRCULATION OF BLOOD IN THE ARTERIES 51 



CIRCULATION OF BLOOD IN THE ARTERIES 



The vessels that carry 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 carrying 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 supply all parts 

 of the body, is given off from the left ventricle. Just at its origin, be- 

 hind the semilunar valves, this vessel has three sacculated pouches called 

 the sinuses of Valsalva. Beyond this point the vessels are cylindrical. 

 The arteries branch, divide and subdivide, until they are reduced to micro- 

 scopic 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 vessels that empty into the capillary system. 

 So long as a vessel gives off no branches, its calibre does not progres- 

 sively 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 verte- 

 bral arteries, which do not diminish in calibre until they enter the fora- 

 men magnum. It has long been remarked that the combined calibre 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 calibre of which is less than the calibre of the abdominal aorta. 



Usually the arteries are so situated as not to be exposed to pressure 

 and consequent interruption of the current of blood ; but in certain situa- 

 tions, as about some of the joints, there is necessarily some liability to 

 occasional compression. In certain parts, 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, com- 

 pression 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 



