INTRODUCTION. 7 



setting free energy so as to move itself, and by reason of its 

 sensitiveness so directing that energy as to produce a movement 

 suitable to the conditions of its surroundings, has at the same 

 time to bear the labour of taking in raw food, of selecting that 

 part of the raw food which is useful and rejecting that which 

 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 

 speaking 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. Bays 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 impulses 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 surroundings 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 to 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 production 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 labour which falls upon 

 each physiological unit of the amoeba. They are not presented 



