ARTERIES. clxvii 



occupies an intermediate position between the origin and termination of 

 each, and the capillaries connect the (lark and the red sets of vessels together 

 at their extremities, and serve as the channels through which the blood 

 passes from the one part of the sanguiferous system to the other, and in 

 which it undergoes its alternate changes of colour, since it becomes dark as 

 it traverses the systemic capillaries and red again in passing through those 

 of the lungs. 



ARTERIES. 



These vessels were so named from the notion that they naturally 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. The degree of deviation 

 of a branch from the direction of the trunk was supposed to affect the force 

 of the stream of blood, but Weber maintains that it can produce little or 

 no effect in a system of elastic tubes maintained, like the arteries, in a state 

 of distension. 



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 Mr. Hunter 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, as the area of the arterial system increases as its vessels divide, 

 the capacity of the smallest vessels aud capillaries will be greatest ; and, as 

 the same rule applies to the \eins, it follows that the arterial aud venous 

 systems may be represented, as regards capacity, by two cones whose apices 

 (truncated it is true) are at the heart, and whose bases aie 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. 



When arteries unite they are said to anastomose or inosculate. Anasto- 



m 2 



