408 THE COMPLETE GRAZIER. BOOK in. 



near the base being 7 inches, and its lateral diameter 5 to 5J inches. 

 Its capacity varies from a pint to a pint and a quarter on each side. 

 Its empty weight is about 6| lb., but it is much greater in well-bred 

 than in under-bred animals. 



The auricles receive blood, whence it passes into the ventricles, the 

 muscular contractions of which drive the blood out of the heart. 

 Impure blood derived from all parts of the body except the lungs is 

 poured by the vense cavse into the right auricle, whence it passes into 

 the right ventricle, and is driven thence along the pulmonary artery 

 into the lungs, where it loses carbonic acid gas and water vapour, 

 gains oxygen, and changes colour from dark (almost black) to scarlet. 

 The pulmonar}*- arteries collect the purified blood from the lungs and 

 pour it into the left auricle, whence it passes into the left ventricle, 

 from which it is driven out through the aorta, which distributes 

 it to all parts of the body except the lungs. Foul and impure, it 

 again finds its way to the right side of the heart, and so the circu- 

 lation goes on. 



Where the auricles open into the ventricles, and where these latter 

 empty into the great arteries, valves are attached. They work in such 

 a fashion as to aid the flow of the blood in the proper direction, and 

 impede its course in the opposite direction. Most of the veins have 

 valves which work on a similar principle. 



Just as the arteries break up into narrower arteries and ultimately 

 into capillaries, so do these latter become at length confluent into 

 small veins, which eventually merge into large veins. As a rule the 

 veins are more superficially seated than the arteries, and not un- 

 commonly one artery is paralleled by a couple of veins. A prominent 

 superficial vein is the jugular, corresponding with the more deeply 

 seated carotid artery. The carotids, on either side, carry blood to the 

 head, the jugulars take it back towards the heart. The jugular vein 

 can easily be felt in the channel extending the length of the horse's 

 neck. To "fill the jugular," press that portion of it nearest the 

 heart, and the vein will at once become " knotted," owing to the 

 valves floating across the vessel and preventing the blood from flowing 

 back again into the head. The pulse is the shock communicated by 

 the contraction of the left ventricle ; it can be felt in the arteries, but 

 is lost ere the blood reaches the veins. The normal pulse in the horse 

 is about 36 per minute, in the ox 55, and in the sheep 75. 



THE NERVOUS SYSTEM. The brain and spinal cord, constituting the 

 cerebro- spinal nervous axis, are securely lodged in the bony cavity, or 

 canal, formed by the skull and the vertebrae. The spinal cord, which 

 extends along the "backbone " to the sacral region, may be regarded 

 as an attenuation backward of the tissues of the brain ; or, conversely, 

 the brain may be looked upon as an enormous development anteriorly 

 of the spinal cord. The aperture at the back of the skull through 

 which the brain becomes continuous with the spinal cord is called the 

 foramen magnum, or the occipital foramen. This region of the 

 cerebro-spinal nervous axis is termed the medulla oblong ata. Cranial 

 and spinal nerves are distributed in pairs from the axis to the various 



