192 LOCOMOTION. 



CHAP. VII. 



of the spine, the voluntary power of the sphincters is lost, and the 

 faeces and urine pass involuntarily. But this is no proof, as is 

 commonly imagined, that the ordinary contraction of the sphincter 

 is an active one,, performed in obedience to a continuous nervous 

 stimulus. The difference is, that it can now induce no active con- 

 traction through the nerves, to counteract temporarily, and in 

 obedience to the will, the active contractions of the parts above, 

 which are not under the influence of volition, and are not paralysed. 

 Hence, whenever the faeces are driven against it, it gives way, 

 against the patient's will, and (if the sensitive nerves are also para- 

 lysed) without his knowledge. 



Contractions are called peristaltic or vermicular, which advance 

 through a muscle in a slow and progressive manner. When ana- 

 lysed closely, we shall find that they are only a variety of the active 

 contraction already described. If a number of striped fibres are 

 arranged in a long series, and are contracted in succession (as in 

 caterpillars) , the resulting movement is vermicular : but in the higher 

 animals it is only in the hollow unstriped muscles that this variety 

 of contraction occurs ; and the best example of it is in the alimen- 

 tary canal. On laying bare the intestines of an animal just killed, 

 we observe successive waves of contraction advancing down the 

 tube, and urging its contents along. They appear to be rendered 

 more active by the contact of the cold air ; but may be re-excited, 

 when they have almost subsided, by irritation of the sympathetic 

 ganglia, from which the muscles are supplied with nerves. If a 

 single point of the intestine be touched, a contraction presently 

 occurs there, which moves onwards to a considerable distance, and 

 is often succeeded by others spontaneously arising. 



It is impossible not to remark the close similitude between these 

 contractions and those visible by the microscope in the striped element- 

 ary fibre. We have here on a large scale the wave-like character 

 there exhibited. A contracting voluntary muscle exposed to view 

 exhibits a tremulous motion, and it may be a question how far this 

 may depend on numerous contractions strictly vermicular, affecting 

 successive sets of fibres, but prevented, by their irregularity and 

 want of coincidence through the whole muscle, from appearing so 

 to the eye. When the pectoral muscle is struck, a knot-like con- 

 traction often moves off in a slow manner in the direction of the 

 fibres. Peristaltic contraction is coincident in a large number of 

 contiguous fibres ; and its progressive character is more easily per- 

 ceived in consequence of the arrangement of the fibres around a 

 compressible cavity. The contraction appears more sluggish th 



