42 SKETCH OF THE ANATOMY OF THE HORSE. 



five times. Hence we find that, speaking generally, small horses 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) by 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 response 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 loco- 

 motion, are grouped together in a long column which fills the brain cavity and 

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



The sj>i?ial cord, though it is formed of a number of nerve centres, is the 

 chief conducting medium by which impressions received by 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. 



