5S8 



CHAPTER VII. 



THE BLOOD AND BI,OOD DISEASES. 



IN the chapter on the diseases of the digestive organs we saw the manner in which the better 

 part of the food taken into the stomach was reduced from food to chyme, changed from chyme 

 to chyle, taken up by a series of vessels termed absorbents, and finally poured into the veins, 

 there to mix with and be matured into blood. 



Blood is that nutritive, life-giving fluid that courses through every portion of the animal 

 system, and that sustains our bodies, and keeps them in health and heat. In appearance it is a 

 reddish fluid, and about the consistency of milk. 



It is of a reddish hue, but the blood drawn from an artery is of a bright vermilion colour, 

 while that abstracted from a vein is more of a purplish tint. In the dog and all mammalia there 

 are not only two sets of blood-vessels (the veins and arteries), but there are in reality two hearts. 

 Both joined together are these two hearts, and communicating, and both covered with the same 

 sac-like membrane called the pericardium, which is one of the large serous membranes of the 

 body, and prevents the friction of the heart against the chest or surrounding structures. 



The heart is a muscular and hollow vessel of somewhat oval form in the dog, similar, or 

 nearly so, to that of a man. It rests on the upper part of the sternum or breast-bone. It is 

 divided into four chambers viz., two auricles (a right and a left), and two ventricles (also right 

 and left). 



The heart is constantly contracting and dilating, and this movement, which goes on until 

 life is extinct, and which resembles the motions of a double force-pump, is sufficient to propel 

 the blood through every artery and vein in the body. The arterial or pure life-giving blood 

 leaves the heart by the great aorta, the largest blood-vessel in the body. No sooner, then, has 

 the aorta started upwards a little way from the left ventricle than it begins to give off branches 

 to the fore-quarters and head ; then, passing backwards in the dog's body along the spine, it 

 gives off branches to all the regions and organs of the abdomen, and finally bifurcates into two 

 large branches, one for each hinder extremity. If we follow the course of one of these we shall 

 find it continuing still to ramify and split up into branches, and each branch ramifying in its 

 turn, until the whole ends in a network of capillaries or hair-like arteries, so small that we cannot 

 prick the flesh with the finest needle without dividing some of them. The diameter of these 

 capillaries is from yjW to -rgVo of an inch. 



And in what do the capillaries end ? They end by anastamosing with veins of their own 

 size, capillary veins. 



We thus trace the arteries down to their capillaries ; to these capillaries the several arteries 

 have carried oxygenated blood, and by means of this blood the system is nourished ; but when 

 the capillaries have done their work, and the effete matter has been taken up into the veins, the 

 blood is no longer pure it can no longer support life until it is taken back to the heart, and 

 thence to the lungs to be purified. Where, then, the arteries end the veins begin, and as the 

 course of the arteries was from the heart, so is the course of the veins towards that centre organ. 



