THE VASCULAR SYSTEM 



paths of a given district, due to a shifting of the centers of 

 population, and the development or decay of any points of 

 human interest. Changes like these set the traffic now over 

 one, now over another, series of roads, which increase or de- 

 crease in width and degree of development in exact propor- 

 tion to this use, certain ones becoming highways and others 

 lanes, solely through the functional importance of the locali- 

 ties which they connect. Even the atrophied rudiments have 

 their counterpart in the ancient roads, entirely overgrown and 

 lost to all save the antiquary. 



The branches of the aorta posterior to the arterial gill- 

 arches and their derivatives are sufficiently similar in all ver- 

 tebrates to be easily recognized, but it may be said in general 

 that, as is the case with other systems, these branches show 

 many more indications of metameric arrangement in the lower 

 vertebrates, and are accordingly more numerous. Instances 

 of this are seen in the numerous lateral and dorsal branches 

 which supply the muscles of the body wall and are segmen- 

 tally arranged in fishes and amphibians, while in higher forms 

 their number is much reduced, forming the intercostal and 

 lumbar arteries. It is again strikingly shown in the mesen- 

 teric arteries which, in lower forms, are very numerous and 

 suggest a metameric series, while in higher forms they are 

 collected at their origin into a common trunk (Fig. 95). 



The relative size of the various branches varies directly 

 with that of the parts which they supply, a fact especially 

 noticeable in the case of the subclavian and iliac arteries, which 

 are small and unimportant in fishes, with small lateral fins, but 

 which become excessively developed in connection with the 

 hypertrophy of one or both pairs of limbs. The caudal aorta, 

 like the other elements of the tail, retains its primitive charac- 

 ter and gives off metamerically arranged branches in the case 

 of well-developed tails, in which the other parts are sufficiently 

 emphasized to allow it. In Man the caudal artery becomes 

 reduced to the insignificant arteria sacralis media, in which the 

 earlier anatomists failed to see the continuation of the aorta. 

 This is in part due, however, to the enormous development 



