HEART AND LUNGS. 47 



infer that the blood of the former passes more frequently during a given 

 space of time through the lungs, than that of the latter. The truth of 

 this supposition is fully borne out in practice ; for we may observe, on 

 an average, that the pulse of a heavy cart-horse beats about thirty-five 

 times a minute ; that of a small pony, about forty-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. — The nervous system of the horse is the 

 power which stimulates and directs the action of his muscles, and 

 is the source of his mental capacity. We may divide it 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 dispatched. The nerves {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 (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 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 spinal cord, though it is formed of a number of nerve centres, 

 is the chief conducting medium by which impressions received by the 



