160 CIRCULATORY SYSTEM. 



the former, without also having the latter; whereas, in animals 

 whose circulating fluid has not red globules, but is a mere serum, 

 the entire destitution of regular blood vessels is very common, 

 and their circulation, if the name be deserved, consists simply 

 in the transmission of moisture from one pore to another, as oc- 

 curs in a rag or in a sponge, by mere capillary attraction. Such 

 animals form a numerous class in the chain of organized beings, 

 and have a gelatinous consistence. 



A remarkable feature in the vascular system, both arteries 

 and veins, is the disposition of trunks to run into one another; 

 or, in other words, to form an anastomosis, whereby, if the 

 blood should be cut off by one route, it may still be supplied 

 through another. These communications are frequent in the 

 head, in the neck, in the thorax, in the abdomen, and in the 

 extremities; they exist, indeed, wherever the blood vessels do, 

 and become more numerous as the blood vessels are smaller, or 

 more removed from the centre of the circulation. It is unne- 

 cessary here to specify instances, as the more remarkable ones 

 will be mentioned at a proper time. But some estimate may 

 be made of their importance, and of the facility of communica- 

 tion established by them, when it is remembered that cases 

 have occurred of obstructed aorta, without the circulation ceasing 

 in the parts of the body beyond it: the same has occurred to the 

 venae cavee, and to the thoracic duct also.* 



The extreme vascular ramifications are called Capillaries, 

 (Vasa CapiHaria,) and they form the connexion between the 

 arteries and veins; or, by being intermediate to the two, they are 

 the ultimate terminations of the arteries, and the commencing 

 roots of the veins. From the extreme tenuity of these vessels, it is 

 impossible to indicate where the arteries terminate and the veins 

 begin; yet their continuity with the capillary system has been 

 repeatedly demonstrated, by throwing injections from the one 

 into the other system, and by microscopical observations made 

 on the transparent parts of living animals, as the mesentery and 

 web foot of frogs, and the tail of fishes. These facts are suffi- 

 ciently substantiated by the observations of Malpighi, Lewen- 



* B^clard, Anat. Gen. 



