INTRODUCTORY. 7 



matter. Both forms of irritable matter are separated, by long 

 tracts of indifferent material, from those contractile tissues through 

 which they chiefly manifest the changes going on in themselves. 

 Hence the necessity for long strands of eminently irritable tissue 

 to connect the skin and contractile tissues as well with each other 

 as with the automatic centres. Similar strands are also needed, 

 though perhaps less urgently, to connect the other tissues with 

 these and with each other. To the vascular bond there must be 

 added an irritable bond, along the strands of which, impulses set 

 up by changes in one or another part, may travel in determinate 

 courses for the regulation of the energy of distant spots. In other 

 words, part of the irritable tissues must be specially arranged to 

 form a coordinating nervous system. 



Still further complications have yet to be considered. In the 

 life of a minute homogeneous amoeba, possessing no special form 

 or structure, there is little scope for purely mechanical operations. 

 As however we trace out the gradual development of the more 

 complex animal forms, we see coming forward into greater and 

 greater prominence the arrangement of the tissues in definite 

 ways to secure mechanical ends. Thus the entire body acquires 

 particular shapes, and parts of the body are built up into mechan- 

 isms, the actions of which are to the advantage of the individual. 

 Into the composition of these mechanisms or ' organs ' the active 

 fundamental tissues, as well as the passive or indifferent tissues, 

 enter ; and the working of each mechanism, the function of each 

 organ, is dependent partly on the mechanical conditions offered by 

 the passive elements, partly on the activity of the active elements. 

 The vascular mechanism, of which we have just spoken, is such 

 a mechanism. Similarly the urgent necessity for the access of 

 oxygen to all parts of the body, has given rise to a complicated 

 respiratory mechanism ; and the needs of copious alimentation, to 

 an alimentary or digestive mechanism. 



Further, inasmuch as muscular movement is one of the chief 

 ends, or the most important means to the chief ends, of animal 

 life, we find the animal body abounding in motor mechanisms, 

 in which the prime mover is muscular contraction, while the 

 machinery is supplied by complicated arrangements of muscles 

 with such indifferent tissues as bone, cartilage, and tendon. In 

 fact, the greater part of the animal body is a collection of mus- 

 cular machines, some serving for locomotion, others for special 

 manoeuvres of particular members and parts, others as an assist- 

 ance to the senses, and yet others for the production of voice, and 

 in man, of speech. 



Lastly, the simple automatism of the amoeba, with its simple 

 responses to external stimuli, is replaced in the higher animals by 

 an exceedingly complex volition affected in multitudinous ways 

 by influences from the world without ; and there is a correspond- 

 ingly complex central nervous system. And here we meet with 



