ARTERIES. 165 



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 inter- 

 costal arteries. The degree of deviation of a branch from the direction 

 of the trunk w^as 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 dis- 

 tension. 



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 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 im- 

 mediately above the point of division. The increase in the joint 

 capacity of the branches over that of the trunk is not in the same pro- 

 portion 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. 



When arteries unite they are said to anastomose or inosculate. 

 Anastomoses may occur in tolerably large arteries, as those of the brain, 

 the hand and foot, and the mesentery, but they are much more frequent 

 in the smaller vessels. Such inosculations admit of a free communi- 

 cation between the currents of blood, and must tend to promote 

 equability of distribution and of pressure, and to obviate the effects of 

 local interruption. 



Arteries commonly pursue a tolerably straight course, but in some 

 parts they are tortuous. Examples of this in the human body are 

 afforded by the arteries of the lips and of the uterus, but more striking 

 instances may be seen in some of the lower animals, as in the well- 

 known case of the long and tortuous spermatic arteries of the ram and 

 the bull. In very moveable parts like the lips, this tortuosity will 

 allow the vessel to follow their motions without undue stretching ; but 

 in otiier cases its purpose is not clear. The physical effect of such a 

 condition of the vessel on the blood flowing along it must be to reduce 



