52 THE CONTRACTILE TISSUES. [BOOK i. 



Some few movements however are carried out by structures 

 which cannot be called muscular. Thus in the pulmonary passages 

 and elsewhere movement is effected by means of cilia attached to 

 epithelium cells ; and elsewhere, as in the case of the migrating 

 white corpuscles of the blood, transference from place to place in 

 the body is brought about by amoeboid movements. But, as we 

 shall see, the changes in the epithelium cell or white corpuscle 

 which are at the bottom of ciliary or amoeboid movements are in 

 all probability fundamentally the same as those which take place 

 in a muscular fibre when it contracts. They are of the nature of 

 a contraction, and hence we may speak of all these as different 

 forms of contractile tissue. 



Of all these various forms of contractile tissue the skeletal 

 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 skeletal muscles are connected with nerves. 

 We have no reason for thinking that they are thrown into con- 

 traction, 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. 



