THE CIRCULATION OF THE BLOOD. O17 
complicated pump; and, if this were so constructed as to pre- 
vent the mixture of blood of different degrees of functional 
value, higher ends would be attained. 
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 
vessels 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. Witha 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 great- 
ly 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-rub- 
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 
