MUSCLES OF ANIMAL LIFE. 273 



fibre. These elementary particles are arranged and united together in the two 

 directions. All the resulting discs as well as fibrillse are equal to one another 

 in size, and contain an equal number of particles. The same particles compose* 

 both. To detach an entire fibrilla, is to abstract a particle of every disc; and 

 vice versa. 1 '' 



368. The elements of Muscular Fibre are bound together, in the perfect 

 condition of the fibre, by a very delicate tubular sheath, which seems to 

 answer to the tube of nervous fibre. This cannot always be readily brought 

 into view ; but it is occasionally seen with great distinctness : thus, when the 

 two ends of a fibre are drawn apart, its 



contents will sometimes separate with- Fig. 63. 



out the rupture of the sheath, which 



then becomes evident; and this, during 



the act of contraction, may sometimes 



be observed to rise up in wrinkles upon 



the surface of the fibre, as spen in Fig. 



66. This sheath is quite distinct from 



the areolar tissue, which binds the 



,. .1? i i Fibre of Human Muscle broken across; the frag- 



fibres mtO fasciculi; and it has been menta connected by the uiitorn sarcolemma. After 



termed, for the sake of distinction, the Bowman. 

 Sarcolemma. Its existence may be de- 

 monstrated in any muscular fibre, by subjecting it to the action of fluids, which 

 occasion a swelling of its contents ; this is especially the effect of acids and 

 alkalies, and may be well produced by the citric and tartaric acids, and by 

 potash. For a time, the Sarcolemma yields to the distension which takes 

 place from within ; but at last it bursts at particular points, and a sort of hernia 

 of its contents takes place, making the existence of a perfect envelop in all 

 other parts quite evident. This membrane is itself perfectly transparent, and 

 has nothing to do with the production of either the longitudinal or the trans- 

 verse stria3. There is no reason to believe that it is perforated either by 

 nerves or by capillary vessels; 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. 



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

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

 flattened against those of adjoining fibres. 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. Their size varies considerably in 

 different classes of animals ; and even in the same animal, and the same mus- 

 cle. 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. 



BIRDS 



Mole 

 Mouse 

 Owl - 

 Chaffinch 

 Heron 



