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exceptions will be found. It may be divided into two parts: the first 

 dealing with the arches, the second with the vasa recta. 



I. 



The number of branches from the left side of the superior mes- 

 enteric artery is very variable; there may be twenty, but there are 

 rarely more than six or seven large ones, the others being, for the 

 most part, insignificant. The first artery is rather small, but it is fol- 

 lowed by from four to six large ones, usually arising near together, 

 which supply the upper half of the intestine. The remainder are 

 smaller. The first two or three divide into an ascending and a de- 

 scending branch, each of which inosculates with the corresponding 

 branch of the neighboring artery, thus forming a single row of arches, 

 from the convexity of which the series of straight vessels goes to the 

 border of the gut. Near the beginning of the small intestine, the most 

 frequent modification of this plan of arches is the appearance of small 

 loops connecting, near their origins, the two arches, into which an 

 artery divides. Thus the parallel vessel as I shall call the com- 

 bined convexities of the distal arches, is formed, chiefly by the sub- 

 divisions of the primary branches, and slightly by these connecting 

 links. 



The next modification to occur is that the branches may give off 

 secondary branches from their sides (I think that it is uncommon for 

 there to be more than one on each side), which subdivide at their 

 ends and insinuate themselves into the system of the primary branches. 

 At the same time the secondary loops of bifurcations become more 

 frequent, and thus, perhaps near the beginning of the second quarter 

 of the gut, we often find an approach to a second tier of arches. A 

 little further along, this is likely to become more unmistakable. The 

 parallel vessel, whence the vasa recta spring, is now rather nearer the 

 gut. I believe that very often a double tier of arches is by no means 

 easy to recognize anywhere. On the other hand, very often below the 

 middle of the Jejuno-Ileum, while the size of the vessels diminishes, 

 the complexity of the arches increases. Sappey declares that it has 

 not been given to him to see four or five tiers, that even three are 

 not constant, and are most likely to be wanting at the beginning and 

 end of the small intestine. I find that the more numerous the tiers 

 the more insignificant the vessels, and that the most complicated 

 systems occur in the lower half. Towards the very end of the Ileum 

 the arrangement is uncertain. There is a good anastomosis between 

 the termination of the superior mesenteric artery and its Ileo- colic 



