324 THE CIRCULATION OF THE BLOOD. [CHAP. XXVili. 



take a similar course, and the arrangement continues to be repeated 

 until the resulting branches are reduced to a very small size, when 

 they pass into the capillary system. This mode of frequent sub- 

 division and anastomosis is seen in the arteries which convey blood 

 to the intestine, the mesenteric arteries. A third form is where 

 two neighbouring arteries communicate by a distinct vessel, which 

 passes from one to the other. By such vessels the remarkable 

 anastomosis at the base of the brain, the circle of Willis, is formed. 

 The anterior cerebral arteries passing upwards and forwards are 

 united by a cross branch, the anterior communicating artery, and 

 the carotid artery on each side is united to the posterior cerebral 

 artery by a branch which passes from before backwards namely, 

 the posterior communicating artery. By this free communication 

 of the arteries in front with those behind, and of those on the right 

 with those on the left, the brain is protected against loss of blood, 

 if any of the main channels of its supply should be stopped. 



The most common form of anastomosis is found in the limbs. Two 

 principal channels convey blood to a limb, as, in the forearm, the 

 radial and ulnar arteries. The branches of these arteries commu- 

 nicate at various points, especially in the vicinity of the joints, 

 and thus, if any impediment occurs in either, the other enlarges, 

 conveys an increased quantity of blood, and even the obstructed 

 trunk beyond the point of obstruction receives a supply by the 

 anastomosing branches. Or the single main artery of a limb, the 

 femoral, or the brachial, by its branches communicates with arteries 

 which, originating from different sources, pass into another portion 

 of the limb. Thus, in the thigh, the circumflex branches of the 

 profunda anastomose with branches which descend from the 

 gluteal, sciatic, and obturator arteries, which are branches of the 

 internal iliac. Hence an obstruction in the femoral, high up, or 

 even in the external iliac, will not deprive the limb of its due 

 supply of blood ; for the arteries just named will convey blood to the 

 branches of the femoral, which arise below. This anastomosis may 

 compensate for an obstruction in the internal iliac near its origin, 

 and by a reflux of blood from the femoral through the profunda 

 arteries supply the lower part of that artery. In the treatment of 

 wounded arteries, the surgeon must always make allowance for the 

 anastomoses in the neighbourhood of the wound. It rarely 

 happens that a single ligature on the cardiac side of the wound is 

 sufficient to guard against secondary hemorrhage ; the anastomotic 

 branches which arise from the main artery above the wound sup- 

 plying the vessel or its subdivisions below, so that the blood finds 



