INTRODUCTION. 27 



The blood traverses the veins also by the power of the heart, but 

 exerted iii a different way. When the ventricle of the heart, after 

 having contracted upon its contents, opens again, it forms a vacuum, 

 into which the neighbouring blood flows by a mechanical principle, 

 viz. the pressure oi' the atmosphere. The fire-engine is supplied with 

 water from the reservoir by the same principle, although the pipe 

 through which it flows may be a quarter of a mile in length. Besides 

 this, where there are muscles by the action of which the vein may 

 be compressed, provision is made that the flow of blood shall be 

 assisted and not retarded ; for there are numerous membranous valves, 

 which open in the natural direction of the current, and would close 

 if the current were to take a retrograde course. All the veins con- 

 nected with the muscles are abundantly supplied with such valves, 

 so that, by every contraction of the muscle, or motion of the limb, 

 the blood is forced on more rapidly in its natural course, and the 

 possibility of retrograding is prevented. This accounts for the in- 

 creased flow of blood on exercise, and the greater rapidity with which 

 the blood escapes in venesection, when the jaw of the ox is moved 

 by introducing the finger into the mouth. 



The Organs of Respiration. — Respiration is so absolutely essential 

 to the life of quadrupeds, that, if it is suspended for a few minutes, 

 the animal dies suftbcatcd. The act of respiration is the alternate 

 reception and expulsion of air into and from the lungs, and, during 

 which, the blood traverses a set of vessels in the lungs, where it is 

 exposed to the action of this air, and changed by it, and rendered 

 capable of supporting animal life. The air which is to effect a salu- 

 tary change in the blood is received partly through the mouth, but 

 mostly through the nostril in cattle, and enters 



The Windpipe^ a long tube situated in the fore part of the neck, 

 and leading from the back of the mouth to the lungs. On the top of 

 it is a triangular cartilaginous substance, which permits the passage 

 of the air either way, but closes the mouth of the windpipe when the 

 animal swallows its food, and so prevents any substance from getting 

 into this tube, and annoying, or perhaps destroying, the beast. 



The windpipe consists of numerous circular rings formed of dense 

 cartilage, and these are connected together by a strong ligamentous 

 substance which is very elastic : this peculiar structure not only 

 renders the windpipe very flexible, but keeps it constantly open. 

 The whole passage is lubricated with a viscid fluid, secreted from 

 the membrane lining its internal surface. 



Immediately before it arrives at the lungs, the windpipe divides 

 into two distinct tubes, and these, as soon as they enter into the 

 lungs, subdivide until they are too minute to be traced by the naked 

 eye, and at length terminate in an innumerable series of minute cells. 

 Upon the membrane lining these cells the vessels which have con- 

 veyed the venous blood to the lungs ramify, and there is nothing 

 interposed between the air and the blood but the membrane forming 

 the cells, and the thin covering of the blood-vessels. 



