THE SCHEMA 161 



waves, it is found that the pressure in that part of the system shows 

 a gradual and uniform rise after the filling of the syringe, or, in other 

 words, the outflow through the high peripheral resistance is uniform, 

 though the inflow into the arterial tube t is intermittent. 



It is while working in this latter manner that the schema very 

 closely reproduces the conditions as found in the circulatory system 

 during life. The rhythmic emptying of the syringe into the elastic 

 tube t, which is filled with fluid exerting a high pressure upon its 

 walls, reproduces the emptying of the left ventricle into the over- 

 distended and elastic aorta. The relative distribution of the fluid is 

 the same in both cases, the arterial tube of the schema being over- 

 filled, and its elastic walls stretched, whereas on the venous side the 

 reverse is the case. The most important particular, however, in 

 which the schema reproduces the conditions found in the circulatory 

 mechanism is the conversion of an intermittent flow at the commence- 

 ment of the arterial tube into a continuous flow through the venous 

 tube. The two factors which lead to this result are : — 



(1) The existence of a high peripheral resistance ; for when that 

 resistance was absent, i.e. when the tube c was open, the flow through 

 the venous tube was intermittent ; and 



(2) The elasticity of the walls of the arterial tube t. 



The influence of the latter can be best exemplified by considering 

 what would happen if the tube were replaced by one whose walls 

 were rigid. In such a case, as the fluid is incompressible and the 

 tube, being rigid, cannot be expanded to hold a larger volume, any 

 fluid forced into the commencement of the tube necessitates the 

 ejection of exactly the same volume of fluid from the opposite end at 

 exactly the same instant. Hence, if the inflow to such a tube be 

 intermittent the outflow must also be intermittent. 



There is another point of view from which we may advantageously 

 consider the flow of fluid in the schema, namely, by studying the 

 amount of work and the way in which it is applied in setting up the 

 flow, and on the other hand the utilisation of that store of energy. 

 The source of energy is the pumping of the syringe : the loss is 

 caused by the friction the fluid encounters in its flow both against 

 the walls of the tubes and at successive layers within the fluid itself 

 where it is moving at different velocities. 



We may, for our present purpose, leave out of account the modifi- 

 cations caused by gravity, for as the fluid is ultimately returned to 

 the same level, they on the whole counterbalance one another. 

 The energy of each emptying of the syringe is at once transferred to 

 the fluid, and is there represented by the velocity imparted to it, and 

 by the pressure set up in it. As soon as the fluid is forced into the 



M 



