524 



MUSCULAR MOTION. 



Independently of the immense disadvantage 

 at which the hypothesis in question supposes 

 the force to act, (viz. either between the par- 

 ticles at the retiring angles only of the zig-zags, 

 or between the distant angles themselves,) it 

 seems quite inconsistent with the able experi- 

 ments of Schwann, which show that the power 

 of a muscle diminishes in a direct ratio with 

 the degree of its contraction. With these 

 experiments, indeed, any hypothesis is at 

 variance which is based on the idea of an 

 attraction between isolated and separate points 

 or particles, as, for example, the sarcous ele- 

 ments, for it cannot be conceived but that such 

 an attractive force would augment in a multi- 

 plying ratio with the proximity of the points 

 attracted. 



2. On passive and active contraction. Pas- 

 sive contraction. Passive contraction is that 

 which every muscle is continually prone to 

 undergo, independently of stimuli, and by the 

 mere quality of its tissue. The muscles are 

 ever kept on the stretch by the nature of their 

 position and attachments, and cannot have their 

 ends so approximated by attitude or other- 

 wise, as that their tendency to shorten them- 

 selves shall cease. If, for example, the rectus 

 muscle of the thigh have its extremities brought 

 as near together as can be effected artificially 

 by posture, they would yet be found to ap- 

 proach still nearer on being freed from their 

 attachment to the bones. This tendency to 

 contract has been distinguished by the term 

 retractility, from its being manifested by the 

 retraction that occurs when the belly of a mus- 

 cle is cut across. But, in this instance, the 

 retraction would appear to be in part caused by 

 an active contraction excited by the stimulus of 

 the injury. It has also been styled tonic ity. 

 The passive contraction of muscles continually 

 opposes their elongation by the action of anta- 

 gonists, and restores them when that action 

 ceases. It is that which accommodates them 

 to an attitude artificially given, when no mus- 

 cular effort is required to maintain it. When 

 no active contraction is present in a limb, the 

 passive contraction remains, and being brought 

 to a state of equilibrium in all the muscles by 

 their mutual antagonism, the limb is said to be 

 at rest. This is the general condition during 

 sleep. The passive contractility of muscles, 

 therefore, is being ever exerted, without being 

 attended by fatigue ; there seems no good rea- 

 son for supposing it to be a property different 

 from active contractility ; it is rather the neces- 

 sary condition of that property, in its passive 

 or unstimulated state. Passive contraction is a 

 vital act, for it ceases with the rigor mortis. 



Active contraction. This is the form of con- 

 traction which is attended with those manifes- 

 tations of power or force that specially charac- 

 terize muscle. It is always excited by a sti- 

 mulus, and is always exerted in opposition to 

 another force within the body, which it is able 

 more or less completely to master. The oppo- 

 sing force is generally the passive contractility 

 of antagonist muscles, but it may be the elasti- 

 city of parts, or, in the case of hollow muscles, 

 the resistance of their own contents. Active 



contraction is partial in extent and duration. 

 It requires intervals of rest, being attended with 

 exhaustion of the power which produces it, 

 which exhaustion in the voluntary muscles is 

 attended with the sensation termed muscular 

 fatigue. 



3. Of the differences between the minute 

 movements of muscle in passive and active con- 

 traction. In passive contraction. It is, per- 

 haps, impossible in the higher animals to ob- 

 serve the nature of the microscopic movements 

 occurring during the passive shortening of a 

 muscle ; but in the lower and smaller forms of 

 life this may sometimes be accomplished. It 

 may always appear doubtful, however, whether 

 any contraction that may be here witnessed be 

 entirely of the passive kind, and consequently 

 the movements here noticed are not worthy of 

 implicit reliance. But it is more easy and quite 

 as satisfactory to bring a muscle under inspec- 

 tion, which is still in situ and in equilibrium 

 with its antagonists ; in such, contractile force 

 is still present, though its effects are neutralized. 

 This may be done in various small animals; per- 

 haps the tail of small fish or of the tadpole of the 

 common frog is the best adapted for the purpose. 

 In the latter, when deprived of its thick tegu- 

 ment, I have succeeded in gaining such a view, 

 and have found the contraction to be quite uni- 

 form throughout, the transverse stripes being sta- 

 tionary and equidistant. This is nothing more 

 than might have been expected on a priori 

 grounds. The contraction being the effect of the 

 passive exercise of the property shared equally 

 by all parts of the tissue, would be equal in equal 

 masses, and as the elementary fibres are of pre- 

 cisely equal width and substance from end to 

 end, no part of them could predominate in ac- 

 tion, as long as no special stimulus was applied. 

 It may be concluded, therefore, that passive 

 contraction is attended by a movement abso- 

 lutely uniform throughout the whole mass of 

 an elementary fibre or of a muscle. 



In active contraction. The case is far other- 

 wise in active contraction, as may now be con- 

 sidered proved by a considerable body of evi- 

 dence. 



It might be argued, prior to direct proof, 

 that active contraction, being the answer to a 

 stimulus, must be partial, at least at its com- 

 mencement, since no stimulus can be applied 

 at the same instant to every particle of a 

 muscle. 



Certain features of the contractions witnessed 

 in fragments removed from the body, and ex- 

 amined in water under the microscope, have a 

 close bearing on the present question. It has 

 been already said that such contractions are 

 uniformly partial; but they present two further 

 varieties, either remaining in the part where 

 they first occur, or leaving it as they engage 

 others in the neighbourhood. The accidental 

 circumstances under which the fragments are 

 placed explain these varieties. In the former 

 case the fragments are free to move; their ends 

 approach in proportion to the amount of con- 

 traction, and as there is no force to extend them 

 again when the contractile force ceases to be 

 manifested in them, and advances to fresh parts, 



