130 CIRCULATION OF THE BLOOD. 



We have to consider its exhausting and its forcing action to deal with 

 it just as we should deal with any hydraulic apparatus. In the old 

 times this organ was looked upon as the seat of the thoughts and the 

 passions ; it was the centre of all good and evil, purity and uncleanness, 

 devotion and love. In the modern system the brain has succeeded to 

 the functions which were once imputed to it. 



The heart, then, is no longer, an altar on which flames are burning, no 

 longer the seat of the passions and the source of love. It is a machine, 

 but what kind of a machine ? How great is the admiration we may ex- 

 press at its exquisite construction ! This little organ can execute three 

 thousand millions of beats without a stop ! In the course of a life, such 

 as we sometimes meet with, it has propelled half a million tons of blood, 

 and, though momentarily wasting, has repaired its own waste all the time. 

 The mathematical rhythm of its four moving cavities, the perfect closure 

 of its mitral and semilunar valves, and the regurgitating play of its tri- 

 cuspid, have never failed it. To the eye of the intellect there is nothing 

 lost in transferring it from the regions of metaphor and speculation to 

 the domain of physical science. 



The doctrine of the circulation of the blood was first propounded by 

 Harvey's doc- Dr. HARVEY about two hundred years ago. It originated 

 cuiTtionof the * n t ^ ie discovering of the valves of the veins by Fabricius ab 

 blood. Aquapendente. After many years of discussion, it was re- 



luctantly received by the medical profession. 



In this doctrine the circulation is referred to causes that are purely 

 mechanical, in the strictest acceptation of that term. The contraction 

 of the walls of the heart propels the blood through the arterial tubes, 

 and even through the veins, the direction of its movement being insured 

 by a proper arrangement of valves. 



But when comparative anatomy and physiological botany were more 

 Its imperfec- extensively cultivated, it was seen that this doctrine is insuf- 

 tions. ficient, for the unity of nature forbids us to believe that nu- 



tritious juices are circulated in different tribes of life by different forces. 

 And though it may be that the contractions of that central impelling 

 mechanism regulate the circulation in those organisms which have a heart, 

 what is to be made of those countless numbers which have none ? In 

 this group we find the whole vegetable creation, and a majority of the 

 animal. 



There is a physical principle which has long appeared to me sufficient. 

 Physical prin- Its use in an explanation of the motion of nutritive juices in 

 iSecaptilary organized systems of every class I have taught in the Uni- 

 circulation. " versity for many years. It possesses the advantage of gen- 

 erality, since it is applicable in every case, from the circulation taking 

 place in a closed cell up to that of man. 



