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MUSCULAR MOTION. 



motor nerves, which are thus made to excite 

 contraction by their immediate and topical 

 action on the muscles. Of these the chief are 

 volition, emotion, and impressions carried by 

 the afferent nerves to the nervous centres and 

 involuntarily reflected thence ; but various 

 diseases and injuries of the motor nerves, either 

 at their origin or in their course, and pressure, 

 heat, chemical substances, electricity, gal- 

 vanism, &c. applied to their texture, are to be 

 ranged under the same head. 



The nature of the change thus induced in 

 the motor nerves is entirely unknown. There 

 seems, however, no ground for believing that 

 it differs with the particular stimulus which 

 induces it, and certainly a clear distinction 

 ought ever to be drawn between it and its 

 exciting cause. The nerves, when this change 

 is induced in them, occasion the muscles to 

 which they are distributed to contract. The 

 stimulus they give is an immediate one, and is 

 termed the vis nervosa or the nervous stimulus 

 of muscle. It acts topically on the muscular 

 fibre. The other immediate stimuli are physi- 

 cal and chemical ; they are rarely exerted in 

 the living body, except in the case of the 

 hollow muscles. It has already been stated 

 that trustworthy experiments have lately shown 

 these to be under the influence of motor nerves 

 derived from the spinal marrow, but it seems 

 probable that some at least are normally excited 

 to contract by direct stimulation, to one form 

 of which, that of stretching or distension, they 

 are peculiarly liable from their arrangement as 

 investments to cavities. All muscles, however, 

 may be made to contract by physical and 

 chemical stimuli applied to their fibres. 



The effects of water and mechanical pressure 

 as immediate stimuli have been already alluded 

 to. 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 pro- 

 bable that electrical forces have a like agency. 

 An interesting phenomenon has been pointed 

 out by Dr. Stokes,* which seems to show very 

 clearly that contractions of voluntary muscles 

 may be excited by an immediate stimulus in 

 the living body. In various cases of phthisis, 

 and in others, particularly those attended with 

 emaciation, a sharp tap with the fingers on any 

 muscular part is instantly followed by a con- 

 traction, evidenced by the rise of a defined 

 firm swelling at the point struck, enduring 

 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 intervention of 

 nerves ; for a contraction excited in the latter 

 mode would be diffused over the part to which 

 the nervous twigs irritated were supplied, and 

 would therefore frequently extend to some dis- 

 tance. 



c. On the visible, changes occurring in muscle 



* On Diseases of the Chest, p. 397. See also 

 Dr. Guy, in thn new edition of Hooper's Phy- 

 sician's Vdde Mccum, p. J)2. 



during contraction. 1 . Of the changes essential 

 to the act. A muscle in action becomes shorter 

 and thicker, and it is well ascertained by 

 experiments often repeated that these changes 

 in its relative dimensions are accurately pro- 

 portioned to one another. The whole organ 

 neither gains nor loses in bulk. 



What is true of the organ is true of the 

 tissue in contraction it increases in diame- 

 ter and shortens in a corresponding degree. 

 This is all that can be said in general respect- 

 ing the visible features of this remarkable 

 phenomenon. Late investigations, instead of 

 explaining the manner in which contraction 

 is effected, by shewing its dependence on 

 forces previously understood, have only served 

 to point out the inadequacy of the coarse and 

 mechanical hypotheses which physiologists have 

 been so prone to confide in, and to make it 

 more than probable that they must ever be 

 content to repose upon the fact above stated as 

 the simplest which the most refined microsco- 

 pical analysis can ever disclose. 



The intimate connexion between the nerves 

 and muscles, both in rest and action, and the 

 exquisite organization displayed in the structure 

 of those muscles which are most quick and 

 energetic in their movements, have powerfully 

 contributed to excite the hope that a clue to the 

 discovery of the physical mechanism of con- 

 traction would one day be found. It may be 

 thought, therefore, a subject of disappointment 

 that when at length a close insight into its 

 visible characters has been obtained, and the 

 minutest particles which the best instruments 

 can discern have been brought under observa- 

 tion during their state of activity, the only 

 change that can be appreciated in them is that 

 which was long ago known by accurate experi- 

 ment to occur in the aggregate mass, viz. that 

 they become shorter and thicker. 



All muscle, after systemic death or after 

 removal from the body, undergoes a contraction, 

 termed the rigor mortis, which has received 

 much attention in all that relates to the mode 

 of its approach, its course and duration, and 

 the practical bearings it presents. (See DEATH.) 

 This phenomenon may be varied by the appli- 

 cation of stimuli, and is eminently suited for 

 the display of the minute changes occurring 

 in muscle during its active state. 



The muscle with striped fibres is peculiarly 

 adapted for the display of these changes ; for 

 its texture not being homogeneous, but marked 

 throughout with perfect regularity into spaces 

 or particles so minute as to require to be very 

 highly magnified before they can be even seen at 

 all, the anatomist is provided with the means 

 of detecting movements, which, without this 

 circumstance, must have remained concealed. 

 It is accordingly by the study of this variety 

 of the tissue that the results just alluded to 

 have been obtained. 



When a fragment retaining its contractility 

 is torn up into its elementary fibres, these are 

 seen to undergo a slow movement at certain 

 points, especially where they have suffered 

 violence, as at their broken extremities. This 

 movement consists of a shortening and thicken- 



