ANASTOMOSES OF 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 would 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, 
__ hy the anastomoses of which a complex network is formed ; 
j and from this network, by the reunion of its small vessels, 
