202 COMPARATIVE PHYSIOLOGY. 



run parallel with the rapidity and extent of the chemical 

 changes. 



Certain drugs also modify the form of the muscle-curve very 

 greatly, so that it appears that the molecular action which un- 

 derlies all the phenomena of muscle and nerve (for what has 

 been said of muscle applies also to nerve, if we substitute nerv- 

 ous impulse for contraction) can go on only within those nar- 

 row bounds which, one realizes more and more in the study of 

 physiology, are set to the activities of living things. 



UNSTRIPED MUSCLE. 



This form of muscular tissue is characterized by its long 

 latent period, its slow wave of contraction, and the prog- 

 ress of the contraction being in either a transverse or longi- 

 tudinal direction, a wave of contraction in one cell being cap- 

 able of setting up a corresponding wave in adjoining cells 

 even when no nerve-fibers are distributed to them. It is ex- 

 cited, though less readily, by all the kinds of stimuli that act 

 upon striped muscle. In the higher groups of animals this 

 tissue is chiefly confined to the viscera of the chest and abdo- 

 men, constituting in the case of some of them the greater part 

 of the whole oi;gan. 



The slow but powerful and rhythmical contraction of this 

 form of muscle adapts it well to the part such organs play in 

 the economy. There are variations, however, in the rapidity, 

 force, regularity, and other qualities of the contraction in dif- 

 ferent parts ; thus, it is comparatively rapid in the iris, and ex- 

 tremely powerful and regular in the uterus, serving to produce 

 that prolonged yet intermittent pressure essential under the 

 circumstances (expulsion of the foetus). 



Comparative.— Muscular contraction is relatively sluggish 

 and prolonged among the invertebrates, to which, however, the 

 movement of the wings of insects is a marked exception, some 

 of them having been shown by the graphic method to vibrate 

 some hundreds of times in a second. 



The slow movements of the snail are proverbial. As a rule, 

 the strength of the muscles of the invertebrates is incomparably 

 greater than that of vertebrates, as witness the powerful grasp 

 of a crab's claw or a beetle's jaws. 



These facts are in harmony with the generally slow metab- 

 olism of most invertebrates and the lower vertebrates. 



