276 OF MUSCULAR CONTRACTILITY. 



traction of muscle, has been that of Prevost and Dumas, who stated that they 

 are thrown into a sinuous or zigzag flexure. Recent observations, however, 

 have fully demonstrated the incorrectness of this view ; the improbability of 

 which might have been suspected from the consideration that fibres in this 

 state of flexure could not be imagined to be exerting any force of traction.* 

 Prof. Owen has noticed that,- in the contracted state of the very transparent 

 muscles of some Entozoa, each separate fibre, which may be seen with great 

 distinctness, presents a knot or swelling in the middle, besides being generally 

 thickened ; but that it is simply shortened without falling out of the straight 

 line. Dr. A. Thomson remarked the same thing in the Frog; single fibres, 

 whilst continuing in contraction, being simply shortened, without falling into 

 zigzag line ; and he was led to suspect, from this and other circumstances, 

 that the zigzag arrangement was not produced until the act of contraction had 

 ceased. The recent inquiries of Mr. Bowman have proved most satisfactorily, 

 that, in a state of contraction, there is an approximation of the transverse 

 strise, and a general shortening of the fibre ; and that its diameter is at the 

 same time increased ; but that it is never thrown out of the straight line, except 

 when it has ceased to contract, and its two extremities are still held in proxi- 

 mity by the contraction of other fibres. The whole process may be distinctly 

 seen under the Microscope in a single fibre isolated from the rest ; it is, of 

 course, desirable to select the specimen from those animals in which the con- 

 tractility of the Muscle is retained for the longest period after death, which 

 is particularly the case in Reptiles among Vertebrata, and in most Invertebrata 

 (Mr. Bowman particularly recommends' the Crab and Lobster) ; but the change 

 has been fully proved to differ in no essential degree, in the warm-blooded 

 Vertebrata. The contraction usually commences at the extremities of the 

 fibre ; but it frequently occurs also at one or more intermediate points. The 

 first appearance is a spot more opaque than the rest, caused by the approxi- 

 mation of a few of the segments of some of the fibrillse: this spot usually 

 extends in a short time through the whole diameter of the fibre, and the 

 shading, caused by the approximation of the transverse striaB, increases in 

 intensity. The strise are found to be two, three, or even four times as nume- 

 rous in the contracted as in the uncontracted part, and are also proportionally 

 narrower and more delicate. The line of demarkation between the contracted 

 and uncontracted portions is well defined ; but, as the process goes on; fresh 

 striae are absorbed (aS-it were) from the latter into the former. The contracted 

 parts augment in thickness; but not in a degree commensurate with its 

 diminished length ; so that its solid parts lie in smaller compass than before, 

 the fluid which previously intervened between them, being -pressed out in 

 bullse under the sarcolemma (Fig. 66). The force with which the elements 



Fig. 66. 



Muscular Fibre of Dytiscus, contracted in the centre ; the stria; approximated ; the breadth of the fibre 

 increased ; and the sarcolemma raised in bullee on its surface. After Bowman. 



* By Prevost and Dumas it was imagined that the muscular fibres themselves were 

 passive agents in contraction; and that the real power was given by an attractive force, 

 analogous to or identical with that of electricity, existing between the nervous fibres, 

 which were stated by them to be disposed in parallel rows, transversely to the direction 

 of the muscle. Other Physiologists, however, have shown that this was a hasty assump- 

 tion ; and it is completely disproved by the numerous facts which prove that muscular 

 contractility does not depend on nervous agency ( 380385). 



