

STRUCTURE OF MUSCULAR TISSUE. 307 



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

 straight line. Dr. Allen Thomson remarked the same thing in the Frog; 

 single fibres, whilst continuing in contraction, being simply shortened, without 

 falling into zigzag lines : and he was led to suspect, from this and other cir- 

 cumstances, that the zigzag arrangement was not produced, until the act of 

 contraction had ceased. The inquiries of Mr. Bowman (loc. cit.) 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 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 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 Yertebrata, and in 

 most Invertebrata (Mr. Bowman particularly recommends the Crab and Lob- 

 ster) ; but the change has been fully proved to differ, in no essential degree, in 

 the warm-blooded Vertebrata. The contraction usually commences at the ex- 

 tremities 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 whole 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 uncontracted part ; and are also proportionally nar- 

 rower and more delicate. The line of demarcation 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 part 

 augments 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 bullae under the 

 myolemma (Fig. 96). The force with which the elements of the fibre thus tend 

 to approximate, is evidently considerable ; for if the two extremities be held 

 apart, the fibre is not unfrequently ruptured. This corresponds with the ap- 

 pearances 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 spas- 

 modic action, the striae have been observed to approximate so closely as to be 



Fig. 96. 



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

 of the fibre increased ; and the myolemma raised in bullae on its surface. 



scarcely distinguishable. When the contraction is not very decided, the dark 

 and elevated spots appears to play like a wave along the fibre, before it in- 

 volves the whole diameter in any part (Fig. 97, B); and even when consider- 

 able traction is being exercised, there is continual interchange in the elements 

 by which it is effected the disks at one end of the contracted part receding 

 from each other, whilst at the other end new disks are being received into it. 

 303. The foregoing description is chiefly derived from the appearances pre- 



