184 LOCOMOTION. [CHAP. vn. 



and appears to be occasioned by those movements of the neighbour- 

 ing fibres upon one another, with which the partial contractions 

 must be attended in their incessant oscillations. 



The other phenomenon is one whose existence has been recently 

 ascertained by MM. Becquerel and Breschet,* viz. that a muscle 

 during contraction, augments in temperature. They have found this 

 increase to be usually more than 1 Fahr.; but sometimes, when 

 the exertion has been continued for five minutes, (as the biceps of 

 the arm, in sawing a piece of wood,) it has been double that 

 amount. This development of heat may be in a great measure 

 attributable to, and even a necessary consequence of, the friction 

 just alluded to. 



Thus it would appear, that in active contraction there is a dis- 

 turbance of the state of equilibrium, or rest, by the application of a 

 special stimulus to certain portions only of each fibre ; by which 

 first these portions, then others in succession, are made to contract 

 strongly, and to pull on the extremities of the fibre through the 

 medium of the parts not so contracted. The contractions undulate 

 along the fibre from the point stimulated, and there is always a con- 

 siderable part of each fibre uncontracted. This will account for the 

 remarkable fact, that detached fragments of the voluntary fibre will 

 contract by two-thirds of their length though an entire muscle, in 

 its natural situation, cannot shorten by more than one-third. This 

 great capacity of contraction in the tissue would be without a pur- 

 pose, if it were not that it only admits of momentary exertion, and 

 therefore requires that in the organ successive parts should take up 

 the act, and by so doing, render it, as a whole, continuous. In an 

 active fibre the contracting parts are continually dragging on those 

 in which the contractile force has j ust subsided, and which intervene 

 between them and the extremities of the fibre. These are thereby 

 instantly stretched, and come to serve the temporary purpose of a 

 tendon ; but one which resists extension more by its passive con- 

 tractility than by its mere tenacity. It is these parts which in 

 tetanic spasm suffer laceration ; which happens in consequence of 

 the contraction excited by the vis nervosa being then too powerful 

 to be resisted by the passive contractility. 



The preceding account of the minute changes occurring during 

 contraction rests on data furnished by the striped form of muscular 

 fibre ; but there is nothing contained in it which seems at variance 

 with the little that is positively known regarding the contractions of 



* Recherches sur la Chaleur Animale. Archiv. du Mus6uin, torn. i. p. 402. 



