190 THE VITAL FUNCTIONS. 



every part of the frame, are name^d, from their being finer 

 than hairs, capillary vessels. 



After the blood, thus transmitted to the different parts of 

 the body by the arteries, has supplied them with the nourish- 

 ment 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 directed 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 Venae cavas, (c,) which 

 pour their accumulated torrent of blood into the general re- 

 servoir, the heart; entering first into the auricle (D,) and 

 thence being carried forward into the ventricle (E,) which 

 again propels 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 reconveying 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 microscope, in the trans- 

 parent membranes of frogs or fishes. The splendid spectacle, 

 thus brought within the cognizance of our senses, of unceasing 

 activity in the minutest filaments of the animal frame, and 

 of the rapid transit of streams of fluid, bearing along with 

 them minute particles, which appear to be pressing forwards, 

 like the passengers in the streets of a crowded city, through 

 multitudes of narrow and winding passages, can never fail, 

 when first beheld, to fill the mind with astonishment;* a feel- 

 ing which must be exalted to the highest admiration, on re- 

 flecting that what we there behold is at all times going on 



* Lewenhoeck, speaking of the delight he experienced on viewing- the 

 circulation of the blood in tadpoles, uses the following- expressions: " This 

 pleasure has oftentimes been so recreating to me, that I do not believe that 

 all the pleasure of fountains, or water-works, either natural or made by art, 

 could have pleased my sight so well, as the view of these creatures has given 

 me." Phil. Trans, xxii. 453. 



