OF THE CIRCULATION. 341 



lungs. Other secretions are also disposing of the 

 material of the body ; and although these be ne- 

 cessary to health, it is the function of respiration 

 which is the most directly necessary to life, and 

 which is guarded by pain and anxiety experienced 

 the moment that interruption is begun. 



We may already comprehend how the blood 

 flows in a great circle, taking up the material of 

 the body by the absorbents and veins of the body, 

 and throwing it off by the lungs : and how blood 

 returning from the lungs, purified by exposure to 

 the atmosphere, comes back to the heart to com- 

 plete its circle. We readily conceive also how 

 this pure blood is necessary to all the vital opera- 

 tions, to the nourishment and growth of the body ; 

 but it is not so easy to comprehend the manner in 

 which the force of the circulation ever keeps pace 

 with the condition of the body : active during the 

 exercise of the body, reduced and equable during 

 repose; or how the body generally will have the 

 circulation moderate in degree, whilst an individual 

 part being excited and in action shall be accom- 

 modated with an activity of circulation exactly 

 apportioned to the necessity for it. It is not pos- 

 sible, on mere hydraulic principles, to explain how 

 the blood shall descend to the toes or ascend to 

 the head by one impulse, and yet with a force ex- 

 actly proportioned to the distance and elevation 

 of the member. Nothing is more admirable than 

 the manner in which the heart, as the great en- 

 gine in the centre, has its irritability and power of 

 30* 



