H2 THE FROG CHAP. 



movement is due to the fibre undergoing a sudden shorten- 

 ing in a particular direction and a consequent approxima- 

 tion 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 

 contracting 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 direction, 

 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 con- 

 tractility, or the power of contraction, may be considered 

 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, /.*., 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?- 

 In addition to the transverse striation a fainter longitudinal 

 striation is more or less distinctly visible. 



Each fibre is covered by a delicate membrane (s\ called 

 sarcolemma^ beneath which nuclei (n) occur at intervals. 



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. 



