13.2 PHYSIOLOGY. 



are furnished only with two cavities to the heart, viz. an auricle and 

 ventricle, similar to and answering the same purposes with the right 

 half of the heart in the most perfect animals 1 . To this order belong 

 all the fish with gills ; therefore these may be called Dicoilia. The 

 last order comprehends the animals whose hearts have only one cavity, 

 as is the case in the insect 2 , therefore they may be called Monocoilia. 



Of the Uses of Arteries. 



The arteries may be said to have three uses. The first and most 

 simple is, the conducting, as canals, the blood to all the different parts 

 of the body. For this purpose they are acted upon by the heart, which 

 is the cause of the motion of the blood (the veins would seem to be, in 

 themselves, passive). But as this motion is by jerks and not by a 

 uniform regular motion, the arteries are made elastic, which [makes 

 them] serve, in some measure, as a reservoir ; while their contractile 

 force continues the stream, although not with the same velocity. This 

 is something like [or upon the same principle as] the air-fountains, or 

 jets, which might be made to imitate the elastic force of the arteries, 

 such as this figure ; but as there is a continued elastic canal, which may 



Syringe throwing in 

 i the water alter- 

 nately. 



■ 

 This jet of water will !i!j|| 



be continued, but I Ml 



higher at the sy- ''jM 



stole of the syringe. |Mjj 



be looked upon as a succession of elastic bodies, every one having some 

 effect, it may be reasonably supposed that in the smallest arteries there 

 is very little pulsation, which we find to be the case ; so that the jerk- 

 ing force of the heart is gradually spent, or fades into the continued 

 [force]. All this is simply mechanical. 



The second use of the arteries is the first purpose answered by this 

 motion of the blood, or the disposing of it. This depends on the living 

 and sensitive principles with which the arteries are endowed, joined 

 with a power of voluntary motion in arteries independently of the 

 general will of the animal, and according to the impression made, joined 

 with the disposition or feelings of the arteries at the time. Then such 



1 [Hunt. Preps. Nos. 904-911.] * [lb. No. 979.] 



