CHAP. VII.] STIMULI TO MUSCULAR CONTRACTION. 175 



to be observed under a different form. A particle of foreign matter, 

 as a hair or a piece of dust, may be included by design or accident 

 in the field of the microscope, so as touch the side of a fibre at a 

 single point. When this happens, the fibre will often exhibit a 

 contraction, so plainly limited to the point touched, as to give un- 

 equivocal proof of its being the result of the irritation of pressure. 

 Chemical substances may be seen to act similarly, if they be not so 

 powerful as to destroy the texture of the part : and it is probable 

 that electricity has a like agency. These interesting phenomena 

 may be observed more or less satisfactorily in all animals whose 

 fibres retain their contractility for a sufficient length of time after re- 

 moval from the body ; and the crab and lobster will be found very 

 favourably adapted for the purpose. In many reptiles, and fish 

 also, the steps occur slowly enough to be adequately scrutinized. 



The facts in question can admit only of one explanation, if it be 

 conceded that the muscular tissue has been here separated from the 

 nervous : and certainly that separation has been effected, unless the 

 nervous tubules send off from their terminal loops a set of fibrils 

 which penetrate the sarcolemma, and diffuse themselves through 

 the contractile material within : a supposition for which there exists, 

 at present, no foundation in the observations of the most diligent 

 investigators of this subject. They will, therefore, probably, be re- 

 garded as conclusive proof that contractility is a property inherent 

 in the very structure of muscle, and capable of being excited to 

 action independently of the instrumentality of nerves. 



An interesting phenomenon has been pointed out by Dr. Stokes, 

 which, when illustrated by the foregoing observations, we may 

 safely consider as an example of contraction in the living body in 

 answer to a physical stimulus. In various cases of phthisis, and, 

 indeed, in all cases attended with emaciation, a sharp tap with the 

 fingers on any muscular part is instantly followed by a contraction, 

 and by the rise of a defined firm swelling, at the point struck, en- 

 during several seconds before it gradually subsides. This is often 

 so prominent as to throw a shadow along the skin, and for the 

 moment it might almost be mistaken for a solid tumour. That it is 

 limited to the point struck is full proof of its being a direct effect of 

 the irritation, and not produced through the medium of nerves ; for 



crimping fish consists in dividing the muscular fibre before it has become 

 rigid, and immersing it in spring water. A small part treated in this manner 

 contracts and hardens within five minutes." Mayo, Physiol., p. 38. It ex- 

 hausts contractility by inducing violent contraction, by which the fibre is 

 often disorganized. 



