CHANGES DURING THE CONTRACTION OF SMOOTH MUSCLE 497 



contract by a single induction current, no matter how intense. Such muscle 

 in the body is always associated with the local nervous apparatus which 

 plays an indeterminate part in its activity. 



The ureters and gall-bladder are the parts most difficult to excite by 

 stimuli; they do not act at all till the stimulus has been long applied, and then 

 contract feebly and to a small extent. The contractions of the cecum and 

 stomach are quicker, and still quicker those of the iris and of the urinary 

 bladder. The contractions of the small and large intestines, of the vas 

 deferens, and of the pregnant uterus are yet more regular and more 

 sustained. 



Changes During the Contraction of Smooth Muscle. The dura- 

 tion as well as type of contraction in smooth muscle is very markedly differ- 

 ent from that of voluntary muscle. A contraction in smooth muscle is 



FIG. 334. Enlarged Detailed Drawing of the Nucleus of Smooth Muscle in the 

 Relaxed and in the Contracted State. Intestine of Necturus. Zeiss obj. 2, oc. 8. 

 (Caroline McGill.) 



characterized by a very long latent period, a slowly developed contraction 

 phase, and an extremely delayed relaxation, figure 356, A. The amount and 

 duration of the contractions are dependent upon the strength and duration of 

 the stimulus, though the curve of contraction itself does not in other respects 

 differ sharply from the type of curve of the simple muscle contraction. 



Owing to the apparently different structural type of smooth muscle, es- 

 pecial interest attaches to the changes which occur during its contraction. 

 Caroline McGill has recently re-examined the histological structure and in- 

 vestigated the function of this type of muscle, and we are able to present a 

 figure showing the changes. The longitudinal fibrillae, which are readily 

 stained with iron hematoxylin, show distinct shortening and thickening at the 



