THE CIRCULATION OF THE BLOOD. 215 



Turning to the channels themselves in which the blood 

 flows, a little consideration will convince one that rigid tubes 

 are wholly unfit for the purpose. Somewhere in the course of 

 the circulation the blood must flow sufficiently slowly, and 

 through vessels thin enough to permit of that interchange be- 

 tween the blood and the tissues, through the medium of the 

 lymph, which is essential from every point of view. The main 

 yessels must have a strength sufficient to resist the force with 

 which the blood is driven into them. 



Now, it is possible to conceive of this being accomplished 

 with an intermittent flow ; but manifestly it would be a great 

 advantage, from a nutritive aspect, that the flow and therefore 

 the supply of tissue pabulum be constant. With a pump regu- 

 larly intermittent in action, provided with valves, elastic tubes 

 having a resistance in them somewhere sufficient to keep them 

 constantly over-distended, and a collection of small vessels with 

 walls of extreme thinness, in which the blood-current is greatly 

 slackened, a steady blood-flow would be maintained, as the 

 student may readily convince himself, by a few experiments of 

 a very simple kind : 



1. To show the difference between rigid tubes and elastic 

 ones, let a piece of glass-rod, drawn out at one end to a small 

 diameter, have attached to the other 'end a Higginson's (two- 

 bulb) syringe, communicating with a vessel containing water. 

 Every time the bulb is squeezed, water flows from the end of 

 the glass rod, but the outflow is perfectly intermittent. 



2. On the other hand, with a long elastic tube of India-rvib- 

 ber, ending in a piece of glass rod drawn out to a point as be- 

 fore, if the action of the pump (bulb) be rapid the outflow will 

 be continuous. An apparatus that every practitioner of medi- 

 cine requires to use answers perhaps still better to illustrate 

 these and other principles of the circulation, such as the pulse, 

 the influence of the force and frequency of the heart-beat on the 

 blood-pressure, etc. We refer to a two-bulb atomizer, the bulb 

 nearer the outflow serving to maintain a constant air-pressure. 



We may now examine the most perfect form of heart 

 known, that of the mammal, in order to ascertain how far it 

 and its adjunct tubes answer to a priori expectations. 



The Mammalian Heart. — In order that the student may gain 

 a correct and thorough knowledge of the anatomy of the heart 

 and the workings of its various parts, we recommend him to 

 pursue some such course as the following : 



