22 INTRODUCTION. 



is useless, and of working up the accepted part through a variety of stages 

 into its own living substance that is to say, it has at the same time that it 

 is feeling and moving to carry on the work of digesting and assimilating. 

 It has, moreover, at the same time to throw out the waste matters arising 

 from the changes taking place in its own substance, having first brought 

 these waste matters into a condition suitable for being thrown out. 



8. In the body of man, movements, as we shall see, are, broadly speak- 

 ing, carried out by means of muscular tissue, and the changes in muscular 

 tissue which lead to the setting free of energy in the form of movement are 

 directed, governed, and adapted to the surroundings of man, by means of 

 nervous tissue. Rays of light fall on the nervous substance of the eye called 

 the retina, and set up in the retina changes which induce in the optic nerve 

 other changes, which in turn are propagated to the brain as nervous impulses, 

 both the excitation and the propagation involving an expenditure of energy. 

 These nervous impulses reaching the brain may induce other nervous im- 

 pulses which, travelling down certain nerves to certain muscles, may lead to 

 changes in those muscles by which they suddenly grow short and pull upon 

 the bones or other structures to which they are attached, in which case we 

 say the man starts ; or the nervous impulses reaching the brain may produce 

 some other effects. Similarly sound falling on the ear, or contact between 

 the skin and some foreign body, or some change in the air or other surround- 

 ings of the body, or some change within the body itself may so affect the 

 nervous tissue of the body that nervous impulses are started and travel to 

 this point or that, to the brain or elsewhere, and eventually may either reach 

 some muscular tissue and so give rise to movements, or may reach other 

 tissues and produce some other effect. 



The muscular tissue then may be considered as given up to the produc- 

 tion of movement, and the nervous tissue as given up to the generation, 

 transformation, and propagation of nervous impulses. In each case there 

 is an expenditure of energy, which in the case of the muscle, as we shall 

 see, leaves the body partly as heat and partly as work done, but in the case 

 of nervous tissue is wholly or almost wholly transformed into heat before it 

 leaves the body ; and this expenditure necessitates a replenishment of energy 

 and a renewal of substance. 



9. In order that these master tissues, the nervous and muscular tissues, 

 may carry on their important works to the best advantage, they are relieved 

 of much of the labor that falls upon each physiological unit of the amoeba. 

 They are not presented with raw food, they are not required to carry out the 

 necessary transformations of their immediate waste matters. The whole of 

 the rest of the body is engaged (1) in so preparing the raw food, and so 

 bringing it to the nervous and muscular tissues that these may build it up 

 into their own substance with the least trouble, and (2) in receiving the 

 waste matters which arise in muscular and nervous tissues, and preparing 

 them for rapid and easy ejection from the body. 



Thus to certain tissues, which we may speak of broadly as " tissues of 

 digestion," is allotted the duty of acting on the food and preparing it for the 

 use of the muscular and nervous tissues ; and to other tissues, which we may 

 speak of as "tissues of excretion," is allotted the duty of clearing the body 

 from the waste matters generated by the muscular and nervous tissues. 



10. These tissues are for the most part arranged in machines or mech- 

 anisms called organs, and the workings of these organs involves movement. 

 The movements of these organs are carried out, like the other movements of 

 the body, chiefly by means of muscular tissue governed by nervous tissue. 

 Hence we may make a distinction between the muscles which are concerned 

 in producing an effect on the world outside man's body, the muscles by which 



