170 STRUCTURE OF MUSCLE. 



According to Mr. Bowman* the ultimate fibres are polygonal in shape 

 [fig. 100] from mutual pressure. They are also variable in their size, not 

 merely in different classes and genera of animals and different sexes, but 

 even in the same muscle. For example, the average diameter of the ulti- 

 mate fibre in the human female is 4^4, while that of the male is 3*5, the 

 average of both being 7 J 3 . The largest fibres are met with in fishes, in 

 which animals they average 2 ^ 2 ; the next largest are found in man, while 

 in other classes they range in the following order: insects ? { 9 ; reptiles 

 4^4 ; mammalia B - J T ; birds F J 7 . 



The ultimate fibrils of animal life, according to Mr. Bowman, are beaded 

 filaments consisting of a regular succession of segments and constrictions, 

 the latter being narrower than the former, and the component substance 

 probably less dense. 



Fig. lOO.f Fig.. 1014 



An ultimate fibre consists of a bundle of these fibrils, which are so dis- 

 posed that all the segments and all the constrictions correspond, and in 

 this manner give rise to the alternate light and dark lines of the transverse 

 stria?. The fibrils are connected together with very different degrees of 

 closeness in different animals ; in man they are but slightly adherent, and 

 distinct longitudinal lines of junction may be observed between them ; 

 they also separate very easily when macerated for some time. Besides 

 the more usual separation of the ultimate fibre into fibrils, it breaks when 

 stretched, into transverse sections [fig. 101,] corresponding with the dark 

 line of the striae, aixl consequently with the constrictions of the fibrillaB. 

 When this division occurs with the greatest facility, the longitudinal lines 

 are indistinct, or scarcely perceptible. " In fact," says Mr. Bowman, 

 " the primitive fasciculus seems to consist of primitive component segments 

 or particles, arranged so as to form, in one sense, fibrillae, and in another 

 sense, discs ; and which of these two may happen to present itself to the 

 observer, will depend on the amount of adhesion, endways or sideways, 

 existing between the segments. Generally, in a recent fas, iculus, there 

 are transverse strise, showing divisions into discs, and longitudinal striae, 

 marking its composition by fibrillae." 



Mr. Bowman has observed that in the substance of the ultimate fibre 

 there exist minute " oval or circular discs, frequently concave on one or 



* On the Minute Structure and Movements of Voluntary Muscle. By Wm. Bowman, 

 Esq. From the Philosophical Transactions for 1840. 



f- Transverse section of ultimate fibres of the biceps, copied from the illustrations to 

 Mr. Bowman's paper. In this figure the polygonal form of the fibres is seen, and their 

 eomnosition of ultimate fibrils. 



% An ultimate fibre, in which the transverse splitting into discs, in the direction of the 

 Constrictions of the ultimate fibrils is seen. From Mr. Bowman's paper. 



