746 A MANUAL OF VETERINARY PHYSIOLOGY 



markable vigour, as if to dislodge a fly. The heart may continue 

 to beat for a minute or two, but the respirations cease. As life 

 is being Extinguished the ligamentum nuchce exercises its elastic 

 recoil in the absence of muscular resistance, and the head is drawn 

 back momentarily with slight jerks until the muzzle projects. 



Soon after death rigor mortis appears (see p. 418), and within 

 a short time tympany of the abdomen is apparent in the her- 

 bivora, reaching such a degree in a few hours, especially during 

 warm weather, that post-mortem ruptures of the diaphragm 

 and other viscera are exceedingly common. The explanation 

 of the tympany is the considerable amount of gas generated by 

 the fermentative decomposition of vegetable food. With death- 

 stiffening the flexor muscles of both fore and hind limbs contract 

 in excess of the extensors, so that the heels of the feet are slightly 

 drawn up ; the ears are also drawn back. 



The Influence of Age on Capacity for Work.— It has been 

 pointed out (p. 97) that the horse, unlike man, does not 

 fail in his heart and arteries in consequence of increasing age. 

 It is not, therefore, a matter for surprise that up to a late period 

 of life he does not lose his capacity for work. In this respect he 

 offers a great contrast to man. At forty years of age there are 

 few men capable of undergoing fast muscular work on their own 

 limbs, for not only are the muscles slower in responding, but 

 the effort required is greater. A child may, indeed, execute 

 with ease muscular movements which would produce a punishing 

 effect on a middle-aged man. If it be accepted that a horse of 

 fifteen is comparable to a man fifty years of age, then it is certain 

 no man of fifty can perform the relative amount of work which 

 a horse can at the corresponding age. With increase in age, horses 

 lose elasticity of tread, but not power for work, even relatively 

 fast work. This applies both to the thoroughbred and other classes. 



No explanation of the fact can at present be offered. It may, 

 perhaps, yet be shown that the vertical position of man exercises 

 a strain on the heart and vessels which is absent in the horizon- 

 tally placed animal. Or there may be some actual defect in the 

 skeletal muscles of man, the result of age, which is unknown in 

 the horse. Trotting work soon wears a man out ; the Japanese 

 and Natal coolie, who pulls a light cart containing one or two 

 people, only lasts three years. He then has to look for lighter 

 employment. A horse performing relatively equivalent work 

 would last much longer. 



Horses of sixteen or seventeen years of age undoubtedly show 

 a falling off, not only in pace, but especially in recuperative 

 capacity after prolonged and severe exertion. This does not 

 in any way detract from the general statement, and remarkable 

 fact, that the horse works equally well at all ordinary periods of 

 life, and in this sense does not grow old. 



