198 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



that fibres in this state of flexure could scarcely 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 short- 

 ened, without falling into zig-zag lines : and he was led to suspect, from this 

 and other circumstances, that the zig-zag arrangement was not produced, until 

 the act of contraction had ceased. The recent inquiries of Mr. Bowman 

 have proved most satisfactorily, that, in the state of contraction, there is an 

 approximation of the transverse striae, and a general shortening of the fibre ; 

 and that its diameter is a,, 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 proximity 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 contractility 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 approximation of a few of the dark points of 

 .some of the fibrillae : this spot usually extends in a short time through the 

 Avhole diameter of the fibre; and the shading, caused by the approximation 

 of the transverse striae, increases in intensity. The striae are found to be 

 two, three, or even four times as numerous, in the contracted, as in the un- 

 contracted part; and are also proportionally narrower and more delicate. 

 The line of demarcation between the contracted and uncontracted portions is 

 well defined; but, as the process goes on, fresh stria3 are absorbed (as it 

 were) from the latter into the former. The contracted part augments in thick- 

 ness ; 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 bullae under the myolemma 

 (Fig. 100). The force with which the elements of the fibre thus tend to ap- 

 proximate is evidently considerable ; for if the two extremities be held apart, 



Fig. 100. 



.Muscular fibre of Dytiscus, contracted in the centre; the striae approximated; the breadth of the fibre 

 increased ; and the sarcolemma raised in bulls; on its surface. 



the fibre is not unfrequently ruptured. This corresponds with the appear- 

 ances found in the muscles of persons who have died from tetanus ; for in 

 the ruptured fibres of those muscles, which have been the subjects of the 

 spasmodic action, the striae have been observed to approximate so closely, 

 as to be scarcely distinguishable. When the contraction is not very decided, 

 the dark and elevated spot appears to play like a wave along the fibre, before 

 it involves the whole diameter in any part (Fig. 101, 2) ; and even when con- 



