198 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



333. When the striated fibre, which must be considered as the highest 

 form of Muscular tissue, is more closely examined, it is seen to consist 

 of a delicate tubular sheath, quite distinct on the one hand from the 

 areolar texture which binds the fibres into fasciculi, and equally distinct 



Fig. 58. 



Fig. 59. 



Fasciculus of striated Muscular 

 Fibre, showing at a the trans- 

 verse striae, and at 6, the longi- 

 tudinal striae. 



Non-striated Muscular Fibre; 

 at 6, in its natural state; at a, 

 showing the nuclei after the action 

 of acetic acid. 



from the internal substance of the fibre. This cannot always be brought 

 into view, on account of its transparency ; it becomes most evident, 

 when, as occasionally happens, the contents of the fibre are separated 

 transversely by the drawing apart of its extremities without the rupture 

 of the sheath ; but it may also be sometimes seen rising up in wrinkles 

 upon the surface of the fibre, when the latter is in a state of contraction. 

 This membranous tube, which has been termed the Myolemma, has 

 nothing to do with the production of the striae, these being due, as will 

 be presently shown, to the peculiar arrangement of its contents. It is 

 not perforated either by nerves or capillary vessels, and forms, in fact, 

 a complete 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, in regard to its wrinkled 

 aspect when the fibre is shortened. 



334. Although Muscular Fibres are commonly described as cylin- 

 drical in form, yet they are in reality rather polygonal, their sides 

 being flattened against those of the adjoining fibres (Fig. 62). 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, on which the transverse striae are very beautifully marked. The 

 size of the fibres is subject to great variation, not merely in different 

 classes of animals, but in different species, in different sexes of the same 

 species, and even in different parts of the same muscle. Thus Mr. 



