ANASTOMOSES OP ARTERIES ANEURISM. 229 



circulation through them, the Surgeon is often able to save a 

 limb, or even a life, which would otherwise be sacrificed. 

 Arteries are liable to a peculiar disease, termed aneurism, 

 which consists in a thinning-away, or rupture, of the tough 

 fibrous coat, and a great dilatation of the other coats, so that 

 a pulsating tumour is formed. This change takes place most 

 frequently at the bend of the thigh, the ham, the shoulder, 

 and the elbow ; where the artery, in the working of these 

 joints, often has to undergo sudden twists. The result of the 

 disease Avould be generally fatal, in consequence of the gradual 

 thinning-away of the walls of the tumour, which at last 

 bursts, allowing the blood to escape from the arterial trunk 

 with such rapidity as, if unchecked, to cause almost instanta- 

 neous death. In order to prevent this, the surgeon ties the 

 artery at some little distance above the aneurism, that is, he 

 puts a thread round it, which is drawn so tight as to prevent 

 the passage of any blood to the aneurism. The circulation in 

 the lower part of the limb is at first retarded ; its temperature 

 falls ; and it becomes more or less insensible. But after the 

 lapse of a few hours, the circulation becomes quite vigorous, 

 the pulsations strong, the temperature rises, and the numb- 

 ness passes off; and as the main trunk still continues com- 

 pletely obstructed, this can only have been brought about by 

 the flow of blood through the anastomoses, which must in 

 that short period have undergone considerable enlargement. 

 Examination of the vessels after death shows that this has 

 been actually the case. Even the aorta has thus been tied in 

 dogs, without causing death ; the anastomoses of the branches 

 given-off from its upper part, with those proceeding from the 

 lower, being sufficient to maintain the circulation in the latter, 

 when the current through the main trunk is obstructed. 



264. A very complex series of anastomoses, forming a com- 

 plete network of large tubes, is found in several situations, 

 where it seems desirable that the flow of blood to a particular 

 organ should be retarded, whilst a large amount is to be 

 allowed to pass through. Thus in animals which keep their 

 heads near the ground for some time together, as in grazing, 

 the arteries which supply the brain suddenly divide, on their 

 entrance within the skull, into a great number of branches, 

 by the anastomoses of which a complex network is formed ; 

 and from this network, by the reunion of its small vessels, 



