CHAPTER VIII 



STABLE CONSTRUCTION AND THE CIRCULATORY SYSTEM 



358. The Circulatory System. A knowledge of 

 the use of fresh air is essential before it is 

 possible to realise the necessity for a continued 

 supply of fresh air in the stable. 



The food of the living cells of the body is 

 blood. Blood carries two important kinds of 

 food: one, protein, salts, etc., as explained in 

 Chapter IV. ; the other, oxygen. Oxygen is not 

 very soluble in the watery fluid portion of the 

 blood, so Nature has made use of a red substance 

 called haemoglobin, which has a very great 

 affinity for oxygen, and absorbs a very great 

 amount of it per unit volume compared with 

 any other liquid. In the horse and other large 

 animals, and in man, Nature has not allowed 

 this haemoglobin to mix freely with the liquid 

 plasma (watery fluid) of the blood, but has con- 

 fined it to little carriers (like delivery vans) 

 called red blood corpuscles (erythrocytes) , 

 which, in the horse, measure l-5,000th of an 

 inch in diameter. A large drop of blood contains 

 several millions of these circular, bi-concave, 

 plate-like cells. They carry a large quantity of 

 haemoglobin. Their function is, as stated above, 

 to carry oxygen, which they obtain from the 

 lungs, and whenever they reach a place in the 

 body where there is not much oxygen, i.e. where 

 the pressure is small compared with the pressure 

 in the corpuscle, they give up some of their 

 oxygen. By the time they get back to the lungs 

 they have exhausted all, or almost all, of their 

 oxygen, and there they receive a fresh supply, 

 because the pressure of oxygen in themselves is 

 less than that in the lung capillaries. 



359. The circulation of the blood consists of 

 a continuous flow that is given an impulse every 

 time the heart beats. The heart is a double 

 pumping station, which is divided into left and 

 right, and each side is again divided into the 

 receiving vessel (auricle) and the pump proper 

 (ventricle). The circuit of the blood is as 

 follows : the left ventricle, which is a very 

 powerful muscular box, forces its contents into 

 the large aorta, then into the other large arteries, 

 and thence into small arteries, until, finally, 

 it reaches the minute capillaries that are 

 situated in every portion of muscle, bone and 

 almost every kind of tissue in the body. Here 

 the blood gives up its oxygen. While full of 

 oxygen, blood is bright red in colour, and is 



called arterial. After parting with its oxygen 

 it becomes blue, and is called venous. From the 

 capillaries the venous blood passes into small 

 veins, then larger veins, then into the vena cava, 

 and thence into the right auricle of the heart. 

 Before it reaches the heart the lymph vessels 

 empty the nourishment that they have collected 

 from the intestinal walls into it. This is the 

 other kind of food that the blood carries. 



The venous blood passes from the right 

 auricle, down through a strong, three-sided 

 valve, into the right ventricle, which forces it 

 into the pulmonary artery (the only artery in 

 the body that carries venous blood), which 

 carries it to the capillaries of the lungs ; in these 

 capillaries the red blood corpuscles pick up their 

 oxygen. The arterial blood then passes into the 

 pulmonary vein, and is poured into the left 

 auricle of the heart. From the left auricle it 

 passes through a very powerful two-sided valve 

 into the left ventricle, whence it is again forced 

 through the body. (See P. 71.) 



360. It will, of course, take a number of 

 beats of the heart for a drop of blood to pass 

 around the whole circuit. A heart-beat consists 

 of a complete cardiac cycle, which is the con- 

 traction of the large valves, the contraction of 

 the auricle, and then the contraction of the ven- 

 tricle ; this period, which is called the systole, 

 occupies three-fifths of a heart-beat. Then the 

 auricle and ventricle relax, and cause the heart 

 to refill ; this period is called the diastole, and 

 occupies the remaining two-fifths of a heart-beat. 



361. Respiratory System. We must briefly 

 consider how the capillaries of the pulmonary 

 circulation come in contact with the oxygen. 



Air contains by weight 21 per cent, (a little 

 over one-fifth) oxygen, nearly 79 per cent, nitro- 

 gen, and about one-twentieth of 1 per cent, 

 carbon dioxide (carbonic acid gas), and a trace 

 of ammonia gas. It contains other gases that do 

 not concern us here. 



Every time an inspiration is taken, air passes 

 into the mouth, through the larynx, windpipe 

 (trachea) and bronchial tubes, to the lungs 

 proper. These consist of numerous branches 

 leading off the bronchial tubes, which subdivide 

 until they become very small tubes that ter- 

 minate in minute air sacs. The surface thus 

 exposed to the inspired air is very great. In a 



