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SPECIAL PHYSIOLOGY. 



and chemical kind, occur in the transference of dissolved material by 

 osmosis and dialysis through the body ; in the transmission of sensory 

 and motor impressions along the nervous substance ; in the passage of 

 electrical currents through the tissues ; and, lastly, in the intimate and 

 incessant changes of nutrition. These movements, thus grouped to- 

 gether, are vito-chemical or vito-physical ; and belong to a class in 

 which perhaps, hereafter, even muscular, ciliary, sarcodic and pro- 

 toplasmic movements will find their place and affinities. 



MUSCULAR CONTRACTILITY. 



Contractility, speaking generally, is, as we. have seen, that property 

 by which a living tissue is capable of shrinking in certain directions, 

 so as to undergo a spontaneous change of form. The muscular con- 

 tractility, muscular irritability, vis musculosa, or vis insita, is* possessed 

 by all the forms of muscular tissue, viz., the striped muscular fibre, the 

 plain or unstriped fibre, and the contractile fibre-cells. This most 

 important vital property, when called into play, produces muscular 

 contraction, which does not consist of a shrinking, condensation or 

 contraction of the tissue in all directions, such as is undergone by a 

 mass of iron in cooling, but of an approximation of the particles in 

 some definite direction, viz., in that of the length of the fibre or fibre- 

 cells. Hence the fibre or fibre-cell, whilst it shortens itself, always 

 increases in thickness. 



Contractility is distinguished from mere elasticity by the fact that 

 it is a property of a living tissue only, i. e., a vital property, whilst 

 elasticity is a physical property which persists in a tissue after death 

 until decomposition or desiccation destroys it. Moreover, elasticity 

 merely requires for its exercise that the elastic part should be pre- 

 viously extended, whereas contractility demands the agency of some 

 external exciting cause or influence, which is called a stimulus. Hence 

 the term muscular excitability occasionally used. 



The stimuli capable of exciting the property of muscular contractil- 

 ity are very various. Some are mechanical, such as a weight, a blow, 

 or a scratch with a pointed instrument, or even a sharp knock on a 

 muscle in the living body ; heat or cold, especially sudden changes of 

 temperature, also act as stimuli ; the chemical stimuli are acids, alka- 

 lies, or mineral salts; vegetable irritants also act, such as mustard; 

 electrical stimuli, such as the galvanic current and electrical shocks ; 

 and, lastly, the vital stimuli, originating in or acting through the ner- 

 vous system, such as the reflex, emotional, ideational and volitional 

 stimuli. It is by means of these vital stimuli that the muscular tissues 

 are most frequently excited to contract in the living body. 



These stimuli may be applied either directly to the muscles, or in- 

 directly through the nerves ; thus, when the prepared hind-limb of a 

 frog is so removed from the body, that a portion of the sciatic nerve, 

 in an uninjured state, projects from its upper end, a mechanical, 

 chemical, or electrical stimulus may be applied either directly to the 

 muscles, and will make them contract, or they may be applied indi- 

 rectly to the projecting nerve, with the same, and even with more 



