36 MUSCULAR CONTRACTION. [BOOK i. 



we shall see there is no change of total bulk, this thickening of the 

 fibre by means of the lateral bulging is necessarily accompanied 

 by a shortening of its length. The contraction of muscular tissue 

 iis in fact a limited and definite amoeboid movement in which 

 intensity and rapidity are gained at the expense of variety. 



Besides these movements which are carried out in the body by 

 means of differentiated muscular tissue, there are others brought 

 about by the peculiar structures known as cilia, among which we 

 may include the motile tails of spermatozoa ; and ordinary amoeboid 

 movements are not wanting, being conspicuously shewn by the 

 so-called migrating cells. We may include both these under the 

 heading of contractile tissues. 



Of all these various forms of contractile tissue the skeletal 

 striated muscles, on account of the more complete development of 

 their functions, will be better studied first ; the others, on account 

 of their very simplicity, are in many respects less satisfactorily 

 understood. 



All the ordinary striated skeletal muscles are connected with 

 nerves. We have no reason for thinking that their contractility is 

 called into play, under normal conditions, otherwise than by the 

 agency of nerves. 



Muscles and nerves being thus so closely allied, and having 

 besides so many properties in common, it will conduce to clearness 

 and brevity if we treat them together. 



