NERVOUS SYSTEM OF THE HORSEL 35 



can " stay" better than large ones ; for the power of " staying " is dependent 

 on the capability, possessed by muscles, of retaining for a long time their 

 contractile power. Also, they recover quicker than big horses from the 

 effects of severe work, owing to the fact that repair of worn-out tissue 

 and removal of waste matters from the system is carried on at a faster rate. 

 In fact, they possess more "vitality." Again, the larger the lungs — other 

 things being equal — ^the greater will be the amount of oxygen taken into the 

 blood, and of impurities given off from the blood into the air. 



Nervous System of the Horse. — The nervous system of the horse is 

 the power which stimulates and directs the action of his muscles in locomo- 

 tion, and is the source of his mental capacity. We may regard it as divided 

 into nerve centres and conducting nerves. To employ a well-worn simile, we 

 may look upon a nerve centre as a telegraph station to which and from which 

 messages are sent and despatched. The nerves (the sensory nerves) hj which 

 the horse sees, feels, hears, smells, and tastes, conduct the impressions they 

 receive to some nerve centre, which may do one of three things, (i) It may, 

 in lesponse to the message received, send, on its own authority, by another 

 line of nerves (the motor nerves), an order (or stimulus) to certain muscles to- 

 move. Such a movement will be by reflex action — that is, the impulse will 

 be immediately reflected back. (2) Instead of acting on its own account, it 

 may merely transmit the message on to another and more important nerve 

 centre to decide what answer will be given. (3) It may use a portion of its 

 transmitting power in reflex action, and a part of it in reporting the matter 

 to head-quarters. 



Besides the power which nerve centres have of exciting the muscles to 

 move in response to a stimulus received from the sensory nerves, they can, 

 by their own initiative, make their motor nerves stimulate to movement the 

 muscles which are supplied with these particular motor nerves. 



The chief nerve centres that are connected with the muscles of 

 locomotion, are grouped together in a long column which fills that brain 

 cavity, and spinal canal, and may be divided into the brain and spinal cord. 



The spinal cord^ though it is formed of a number of nerve centres, is the 

 chief conducting medium by which impressions received hj the senses are 

 conveyed to the brain, and is the means by which orders from the brain 

 are transmitted to the muscles of the limbs. 



We may divide the brain into the medulla oblongata, the cerebellum, 

 and the cerebrum. 



The medulla oblongata connects the other two portions of the brain with 

 the spinal cord. It is the nervous centre of the function of breathing. 

 Animals, for purposes of experiment, have had their spinal cord, and the 

 whole of their brain, except the medulla, removed, and yet they have 

 continued to breathe and live. But were the meduUa injured, death from 

 inability to breathe would at once ensue. 



The cerebellum appears to be the organ of musmhr sense and of combined 

 muscular effort By its muscular sense the animal can tell, from experience, 



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