28 LECTURE I. 



out being essential to these modes o£ ac- 

 tion. 



In the human body we see instances of 

 irritabih'ty exerting itself after the manner 

 it does in general in tardigrade animals. 

 If the iris had possessed the ordinary 

 powers of muscles, and none else, it could 

 not have remained, as it is known to do, 

 permanently contracted in a strong light, 

 and permanently dilated in a weak one. 

 Indeed, an anatomist who is fond of tracing 

 structure as connected with function, might 

 readily persuade himself, that there is in 

 the iris a distribution of arteries, similar to 

 that which Mr. Carlisle has demonstrated 

 in the limbs of sloths. We find, however, 

 that sphincter muscles in general have the 

 power of continuing their contraction, 

 though no peculiar distribution of vessels 

 is discoverable in them. In the gall blad- 

 der, the function of which requires this 

 slow but permanently acting irritability, in 

 order to express its contents in small and 

 equal quantities into the bowels, as the di- 

 gested aliment passes into them, we discover 



