360 BLOOD-VESSELS. 



BLOOD-VESSELS. 



The blood, from which the solid textures immediately derive material for their 

 nourishment, is conveyed through the body by branched tubes named blood-vessels. 

 It is driven along these channels by the action of the heart, which is a hollow 

 muscular organ placed in the centre of the sauguiferous system. One set of vessels, 

 the arteries, conducts the blood out from the heart and distributes it to the different 

 regions of the body, whilst other vessels, the reins, bring it back to the heart again. 

 From the extreme branches of the arteries the blood gets into the commencing 

 branches of the veins or revehent vessels, by passing through a network of fine 

 tubes which connect the two, and which are termed, by reason of their smallness, the 

 capillary (i.e., hair-like) vessels, or, simply, the capillaries. 



ARTERIES. 



These vessels were originally supposed to contain air. This error, which had long 

 prevailed in the schools of medicine, was refuted by Galen, who showed that the 

 vessels called arteries, though for the most part found empty after death, really 

 contain blood in the living body. 



Mode of distribution. The arteries usually occupy protected situations ; 

 thus, after coming out of the great visceral cavities of the body, they run along the 

 limbs on the aspect of flexion, and not upon that of extension where they would be 

 more exposed to accidental injury. 



As they proceed in their course the arteries divide into branches, and the division 

 may take place in different modes. An artery may at once resolve itself into two or 

 more branches, no one of which greatly exceeds the rest in magnitude, or it may give 

 off several branches in succession and still maintain its character as a trunk. The 

 branches come off at different angles, most commonly so as to form an acute angle 

 with the further part of the trunk, but sometimes a right or an obtuse angle, of 

 which there are examples in the origin of the intercostal arteries. 



An artery, after a branch has gone off from it, is smaller than before, but usually 

 continues uniform in diameter or cylindrical until the next secession ; thus it was 

 found by Hmiter that the long carotid artery of the camel does not diminish in 

 calibre throughout its length. A branch of an artery is less than the trunk from 

 which it springs, but the combined area or collective capacity of all the branches into 

 which an artery divides, is greater than the calibre of the parent vessel immediately 

 above the point of division. The increase in the joint capacity of the branches over 

 that of the trunk is not in the same proportion in every instance of division, and 

 there is at least one case known in which there is no enlargement, namely, the 

 division of the aorta into the common iliac and sacral arteries ; still, notwithstanding 

 this and other possible exceptions, it must be admitted as a general rule that an 

 enlargement of area takes place. From this it is plain that, since the area of the 

 arterial system increases as its vessels divide, the capacity of the smallest vessels and 

 capillaries will be greatest ; and, as the same rule applies to the veins, it follows 

 that the arterial and venous systems may be represented, as regards capacity, by two 

 cones whose apices (truncated it is true) are at the heart, and whose bases are united 

 in the capillary system. The effect of this must be to make the blood move more 

 slowly as it advances along the arteries to the capillaries, like the current of a river 

 when it flows in a wider and deeper channel, and to accelerate its speed as it returns 

 from the capillaries to the venous trunks. 



