420 THE TISSUES. 



considered it (p. 405). The elasticity doubtless inheres in part in 

 the myolemmata of the muscles, since they resemble elastic tissue. 



Physiological Remarks. 



The use of the muscles depends on the vital property inherent 

 in the muscular fibres contractility, or spontaneous shortening 

 producing motion in the parts (bones especially) with which they 

 are connected. The tendons and aponeuroses only serve to trans- 

 mit the motor force generated in the muscular fibres. Hence the 

 tendons of the flexors and extensors of the fingers and toes are 

 elongated, in order to remove the belly of the muscle to a distance, 

 and secure beauty of proportion in the limbs. During contraction, 

 the fibres shorten in a rectilinear direction, but do not undergo any 

 considerable condensation. Their tenacity is, however, much in- 

 creased ; so that the tendon gives way rather than the belly of the 

 muscle, if rupture occurs from violent contraction as in the tendo- 

 Achillis, &c. Ordinarily, only a part of the fibres in a muscle con- 

 tract at the same time (p. 402). Thus the full strength of a muscle 

 is seldom exerted, and usually only under the*highest excitement 

 as in mania, &c. 



It has been stated that the muscles ordinarily contract in conse- 

 quence of a stimulus communicated to them by the nerves (p. 402). 

 Of the nature and precise mode of their action, we, however, know 

 nothing; though it may be inferred that, in the vertebrata, the 

 nervous influence acts from a certain distance, since the nerve- 

 fibres touch the muscular fibres only at a few points, and never 

 penetrate into their interior (p. 416). 



The intrinsic contractile force of a muscle depends not on its 

 length, but on the number and size of its fibres, or its transverse 

 sectional area. The extent of motion, however, at the point of in- 

 sertion, depends, cceteris paribus, on the length of the fibres, and can 

 never exceed three-fourths to five-sixths of the length of the belly 

 of the muscle, since no individual fibre shortens more than in this 

 proportion (p. 402). When, however, the fibres are short, a com- 

 pensation in respect to the amount of motion of the part to be 

 moved may be secured by having the insertion of the muscle nearer 

 to the extremity of the bone to be moved by it e. g. the semi-mem- 

 branosus, as compared with the semi-tendinosus. The available or 

 effective force of a muscle depends much on its relations to the part 

 into which it is inserted. Most of the muscles are inserted in such 



