

102\ I $*\ ;;'; ?';*\ jStjJ-TJED MUSCLE. [BOOK i. 



along the individual fibre ; in the case of the intestine or ureter, 

 the wave is complex, being the sum of the contraction-waves of 

 several fibres engaged in different phases and is propagated from 

 fibre to fibre, both in the direction of the fibres, as when the whole 

 circumference of the intestine is engaged in the contraction, or 

 when the wave travels longitudinally along the longitudinal coat, 

 and also in a direction at right angles to the axes of the fibres, as 

 when the contraction-wave travels lengthways along the circular 

 coat of the intestine, or when it passes across a breadth of the 

 longitudinal coat. Moreover, it is obvious that the contraction- 

 wave which passes along a single unstriated fibre differs from that 

 passing along a striated fibre, in the very great length both of its 

 latent period and of the duration of its contraction. 



Waves of contraction thus passing along the circular and longi- 

 tudinal coats of the intestine constitute what is called peristaltic 

 action. 



Like the skeletal muscles, whose nervous elements have been 

 rendered functionally incapable (p. 86), unstriated muscles are 

 much more sensitive to the making and breaking of a constant 

 current than to induction-shocks. 



The unstriated muscles seem to be remarkably susceptible to 

 the influences of temperature. Thus the unstriated muscles of 

 the trachea are said not to contract at a temperature below 12C., 

 and are most active at a temperature above 21C. So also the 

 movements of the intestine cease at a temperature below 19C. 



In striking contradistinction to what takes place in the striated 

 muscles, automatic movements are exceedingly common in struc- 

 tures built up of non-striated muscles ; these moreover exhibit a 

 great tendency to rhythmic action. Thus the peristaltic action of 

 the intestine and ureters, and the corresponding movements of the 

 uterus, are at once rhythmic, and largely automatic. What share 

 the nervous elements take in the automatism and the rhythm is 

 uncertain. 



Cardiac Muscles. The most important features of this form of 

 contractile tissue will be studied when we come to deal with the 

 heart. It will be seen that they are intermediate between ordinary 

 skeletal and non-striated muscles. 



Cilia. Ciliary movement consists in the rapid flexion (into a 

 sickle or hook-form) of the cilium and its less rapid return to its 

 previous straight form. The diminished velocity of the return 

 leads to the force of the ciliary action being exerted in the same 

 direction as the flexion. The cause of the flexion seems to be the 

 contraction of the cilium, and that of the return, an elastic reaction. 

 In the lower animals however many'varieties in the mode of move- 

 ment of cilia may be observed. 



Various attempts to explain the movement by the presence of 

 special mechanisms at the base of the cilia have hitherto failed. 

 Some authors have attributed the movement to a' protoplasmic 



