CHAP. XXVIII.] THE STRUCTURE OF VEINS. 325 



its way by a reflex course to the distal part of the trunk of the 

 artery, and to the wound. This is very apt to occur in wounds of 

 the brachial artery at the bend of the elbow. The only and the 

 obvious method of guarding against such an effect of arterial ana- 

 stomosis is to apply a ligature to two points of the artery, viz., 

 below the wound, as well as above it. 



The supply of blood to various segments of the body through 

 different channels, and the free communication of these channels 

 with each other, must be regarded as one of the most beautiful of 

 the various mechanical contrivances in the human body. By such 

 an arrangement a considerable security is obtained against the 

 failure of the nutrition of the limb by the stoppage of one of its 

 channels. And modern surgery is largely indebted to it for one of 

 its most brilliant triumphs. 



The passage of the blood into a limb or organ through various 

 channels serves to distribute it more equally, and to relieve the ele- 

 mentary constituents of the limb or organ from the impetus which 

 the entrance into it of a single large column of blood would 

 occasion. This provision is especially secured for the brain by the 

 subdivision of the four great streams of blood which enter the 

 cranium into several minor ones at the base of that organ, which 

 again undergo extreme subdivision before they penetrate the nervous 

 matter. In animals that hold their heads low, the subdivision of 

 the carotid and of the vertebral arteries is very remarkable, and 

 gives rise to the formation of the different kinds of rete mirabile. 

 The most remarkable instance of the subdivision of arteries prior to 

 the penetration of the tissue they are destined to nourish is that 

 described by Sir A. Carlisle in the Sloth, which seems to be con- 

 nected with the extraordinary power enjoyed by those animals of 

 sustaining muscular action for a lengthened period. 



The anastomoses of the smaller arterial ramifications are also 

 of great importance in many of the organs and tissues, especially 

 under the skin and mucous membranes. Here a membranous expan- 

 sion is supplied by a great number of distinct twigs which form a 

 plexus everywhere continuous, and which again gives rise to other 

 smaller plexuses before the ultimate capillaries are given off. To 

 this form of anastomosis of the smaller arteries may perhaps be 

 ascribed the tendency of some inflammations of membranous 

 parts to be propagated rapidly along an extensive surface, as in 

 erysipelas. In some organs, as the kidney, the arterial twigs have 

 no anastomosis whatever. 



Of the Veins. The veins carry the blood back to the heart from 



