232 LECTURES TO SCIENCE TEACHERS. 



which are displayed by a living animal as such its func- 

 tions as they are called may be divided like those of the 

 inorganic world, into chemical and mechanical. 



As instances of mechanical functions I may mention 

 locomotion, respiration, circulation. All of these consist 

 in the performance of motions which can be registered by 

 methods similar to those we have used for registering the 

 movement of a falling body. All of them are performed 

 by the combined action of various forms of muscular 

 apparatus. 



The vital motions first registered were those of the cir- 

 culation. The first graphic apparatus ever used in phy- 

 siology was, as I have already mentioned, Ludwig's, and 

 the purpose for which he devised it was the measuring and 

 recording the variations of hydrostatic pressure in the 

 arterial system. To enable us to understand this, I must 

 place before you one or two physiological facts. 



I will assume that you are acquainted with the general 

 arrangement of the circulatory system, i.e., that it consists 

 of the heart a pump, and various tubes for the convey- 

 ance of blood, namely, arteries, capillaries, and veins. You 

 probably also know that the arteries act as a distended 

 reservoir, out of which in the forwards direction blood is 

 constantly flowing, into which from behind blood is being 

 pumped at intervals by the heart. Now for all the practical 

 purposes of life what is essential is that blood should flow 

 through the capillaries of every part of our body in a 

 constant stream. What is the cause of this motion 1 

 The obvious answer is, Because the heart pumps the blood 

 on. This is a true but a very imperfect answer. Its incom- 

 pleteness consists in this that instead of giving the imme- 

 diate cause of the phenomenon, you give the more remote 

 one. The immediate cause of the circulation of blood 

 through the capillaries of my finger is not the pumping 

 action of the heart obviously not ; for if I could put the 

 capillaries of my finger under the microscope as I can the 

 web of the frog's foot, I should see that the stream does 

 not keep time with the stroke of the pump, but goes on in 

 a continuous and equable flow. 



In the transparent web of the foot of the frog I can 

 actually see that this is so, for if while I am watching the 

 flow of blood through the capillaries of the web, I suddenly 



