112 THE FROG CHAP. 



colourless blood-corpuscles. Muscular movement is 

 due to the fibre undergoing a sudden shortening in a 

 particular direction and a consequent approximation of 

 its two ends. Ciliary movement is due to the alternate 

 bending and straightening of the cilia ; and the bending 

 of a cilium in a particular direction is caused by the 

 protoplasm of which it is composed shortening or con- 

 tracting on the side towards which it bends. Amoeboid 

 movement is the protrusion and withdrawal of irregular 

 processes of the cell : this results from the protoplasm 

 undergoing a contraction or squeezing in a given direc- 

 tion, as a consequence of which one part of its substance 

 is drawn in and another pushed out. Hence all three 

 kinds of movement are movements of contraction ; and 

 contractility, or the power of contraction, may be con- 

 sidered as a general property of protoplasm. 



Striped Muscle. If a small piece of any of the body- 

 muscles is carefully teased out with the grain, i.e., in 

 the direction of the length of the fibres, so as to break 

 away the connective-tissue binding them together, the 

 fibres, which are much larger than those of smooth 

 muscle, will readily separate from one another, and 

 they will be seen to be long and cylindrical. Under the 

 microscope each fibre shows a delicate transverse 

 striation (Fig. 32), being made up of alternate bright 

 (b) and dim (d) bands or more accurately discs, the 

 fibre being cylindrical set at right angles to its length. 

 Hence the ordinary body-muscles or voluntary muscles 

 are composed of striped muscular fibres. 1 In addition to 

 the transverse striation a fainter longitudinal striaticn is 

 more or less distinctly visible. 



Each fibre is covered by a delicate membrane (s), i 



1 The muscles of the heart, although not under the control of 

 the will, are transversely striated ; but their structure differs ' 

 from that of ordinary striped voluntary muscle. 



