THE ANIMAL MACHINE 19 



tissues themselves without being confined within definite 

 walls except in the main vessels. 



23. Nerve control. — The general function of the 

 nervous system is to exert control over the functions of 

 the other systems, coordinating all movement and regu- 

 lating all vital processes. The strength and resourceful- 

 ness with which this function is exerted is expressed as 

 nerve force, and is usually most marked in the highest bred 

 individuals. Nervousness is no indication of nerve force. 



The whole nervous organization is comparable to a 

 telegraph system, in which the central station receives 

 reports and proceeds to act upon them in communicating 

 instructions, for execution, to other parts, usually remote. 

 Nerve impulses may be sensory or motor, and motor im- 

 pulses may be voluntary or involuntary, depending upon 

 the character of the function involved. For instance, the 

 horse receives a certain pressure from the bit representing 

 an idea in the mind of his rider or driver; the sensory 

 impulse is conveyed to the brain of the horse, from which 

 is sent out the motor impulse which results in the execu- 

 tion of the idea. 



Reproduction 



24. Impregnation. — Copulation is the physical act by 

 means of which the male germ cell, or spermatozoon, is 

 conveyed to the genital tract of the female, there to meet 

 the female cell, or ovum, recently discharged from the 

 ovary. Any prospective breeding animal must be capable 

 of it. The female will only submit to the male during the 

 period of oestrum, or heat, at which time a Graffian follicle 

 in the ovary is maturing and a ripe ovum is hberated. 

 Conception takes place in either the body or the horns 

 of the uterus (Fig. 9), depending upon the species, and its 



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