CIRCULATION IN THE VERTEBRATA. 263 



terruptions to the circulation, which might arise 

 from accidental obstructions in any particular 

 branches of this extended system of canals. 

 The minutest vessels (p p), which in incalculable 

 numbers, pervade every part of the frame, are 

 named, from their being finer than hairs, capil- 

 lary vessels. 



After the blood, thus transmitted to the differ- 

 ent parts of the body by the arteries, has supplied 

 them with the nourishment they require, it is 

 conveyed back to the heart by the veins, which, 

 commencing from the extreme ramifications of 

 the arteries, bend back again in a course di- 

 rected towards the heart. The smaller branches 

 join in succession to form larger and larger 

 trunks, till they are at length all united into one 

 or two main pipes, called the Vence cavcB, (c), 

 which pour their accumulated torrent of blood 

 into the general reservoir, the heart; entering 

 first into the auricle (d), and thence being carried 

 forward into the ventricle (e), which again pro- 

 pels it through the Aorta. The veins are larger 

 and more numerous than the arteries, and may 

 be compared to rivers, which collecting all the 

 water that is not imbibed by the soil, and recon- 

 veying it into its general receptacle, the ocean, 

 perform an analogous office in the economy of 

 the earth. 



The communications of the capillary arteries 

 with the veins are beautifully seen, under the 



