STRIATED MUSCULAR FIBRE. 195 



evident. This membrane is itself perfectly transparent, and has nothing to do 

 with the production of either the longitudinal or the transverse striae. There is 

 no reason to believe that it is perforated either by nerves or by capillary ves- 

 sels ; in fact it seems to be an effectual barrier between the real elements of 

 Muscular structure, and the surrounding parts. That it has no share in the 

 contraction of the fibre, is evident from the fact just mentioned, respecting the 

 condition which it occasionally presents when the fibre is much shortened. 



228. Muscular Fibres are commonly described as cylindrical ; but there is 

 reason to believe that they are rather of a polygonal form, their sides being 

 flattened against those of adjoining fibres (Fig. 97). In some instances the 

 angles are sharp and decided ; in others they are rounded off, so as to leave 

 spaces between the contiguous fibres for the passage of vessels. In Insects, 

 the fibres often present the form of flattened bands. The average diameter of 

 the fibres in Man maybe stated at about l-400th of an inch; being somewhat 

 greater than this in the Male, and less in the Female. Their size varies con- 

 siderably, however, in different classes of animals; and even in the same ani- 

 mal, and the same muscle. The following table gives illustrations of these 

 varieties; the extremes are those met with by Mr. Bowman himself; but 

 other observers speak of dimensions more widely separated. 



Fractions of an inch. 



MAMMALIA 



BIRDS . . 

 REPTILES . 



FISH . . 



INSECTS 



It is interesting to remark, upon this table, that the Muscular Fibre of Rep- 

 tiles and Fishes is upon the whole much larger than that of other Vertebrata, 

 and that its dimensions present the greatest extremes of variation ; whilst in 

 Birds, it is much smaller than in all other Vertebrata, and its dimensions are 

 also less variable. Further, the size of the fibres bears no proportion to that 

 of the animal; for we observe that in the Chaffinch they are larger than in 

 the Owl, in the Cat larger than in the Horse, and in the Frog often larger 

 than in the Boa. Moreover in Insects, the diameter of the fibres is even 

 greater than it is in Mammalia. The average distance of the transverse striae, 

 in the muscular fibre of different animals, is very nearly uniform ; as will be 

 seen from the following table. Between the extremes, however, there is con- 

 siderable variation ; and this will be presently shown to depend upon the con- 

 dition of the muscle, at the time of examination. The distance is not only 

 often different in the same muscle and the same fasciculus, but even in the 

 same fibre in different parts of its length. The figures indicate the number of 

 striae in l-1000th of an inch. The extremes in the same specimen, however, 

 are in no instance so widely apart, as the table indicates for the Class ; the 



