STRUCTURE OP MUSCLE. 143 



animals ; in man they are but slightly adherent, and distinct lon- 

 gitudinal 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, corresponding with the 

 dark line of the striae, and consequently with the constrictions of 

 the fibrillae. 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 themselves to the observer, will 

 depend on the amount of adhesion, endways or sideways, existing 

 between the segments. Generally, in a recent fasciculus, there are 

 transverse striae, 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 both 

 surfaces, and containing, somewhere 

 near the centre, one, two or three minute 

 dots or granules." Occasionally they 

 are seen td present irregularities of form, 

 which Mr. Bowman is inclined to regard 

 as accidental. They are situated be- 

 tween, and are connected with the 

 fibrils, and are distributed in pretty 

 equal numbers through the fibre. These 

 corpuscles are the nuclei of the nucle- 

 ated cells from which the muscular fibre was originally developed. 

 From observing, however, that their "absolute number is far 

 greater in the adult than in the foetus, while their number, rela- 

 tively to the bulk of the fasciculi, at these two epochs, remains 

 nearly the same," Mr. Bowman regards it as certain, that " during 

 developement, and subsequently, a further and successive deposit of 

 corpuscles" takes place. The corpuscles are only brought into 

 view when the muscular fibre is acted upon by a solution of " one 

 of the milder acids, as the citric." 



Muscles are divided into two great classes, voluntary and 

 involuntary, to which may be added as an intermediate and con- 

 necting link, the muscle of the vascular system, the heart. 



The voluntary, or system of animal life, is developed from the 

 external or serous layer of the germinal membrane, and compre- 

 hends the whole of the muscles of the limbs and of the trunk. The 

 involuntary or organic system is developed from the internal or 

 mucous layer, and constitutes the thin muscular structure of the 



H. Mass of ultimate fibres from the pectoralis major of the human foetus, at nine 

 months. These fibres have been immersed in a solution of tartaric acid, and their 

 " numerous corpuscules, turned in various directions, some presenting nucleoli," are 

 shewn. From Mr. Bowman's paper. 



