THE MUSCULAR SYSTEM. 



Muscular Tissue in General. — Contractility, although exhibited to some 

 degree by all living protoplasm, is possessed especially by muscular tissue, the sum 

 of the contractions of such tissue being expressed in motion, the most conspicuous 

 characteristic of all the higher forms of animal life. Muscular tissue represents a high 

 specialization in which contraction takes place along definite lines corresponding to 

 the long axes of the component cells, in contrast to the uncertain contractility occurring 

 within other elements. 



The simplest form of contractile tissue, as seen in some of the low invertebrates, 

 is represented by elements of which the superficial part is related to the integument, 

 the deeper being differentiated into contractile fibres. Although such musculo-epithe- 

 lial cells may form an almost complete contractile layer, the muscular fibres do not 

 exist as an independent tissue. The differentiation of certain cells into definite mus- 

 cular tissue, however, soon appears in the members of the zoological scale, although 

 the existence of a distinct muscular system is deferred until an. adequate nervous 

 system is developed. 



In the higher animals muscular tissue appears in two chief varieties, the striated 

 and non- striated, depending upon the respective histological characteristics of their 

 constituent elements. The former makes up the muscles controlled by the will, and 

 is, therefore, also termed voluntary muscle ; the latter, which constitutes the contrac- 

 tile tissue within the walls of the hollow viscera, blood-vessels and other tubes, acts 

 independently of volition, and is spoken of as involuntary muscle. The last named 

 is sometimes also designated vegetative muscle, since the organs in which it is present 

 are largely concerned in the nutritive processes ; the term animal may be applied in 

 contrast to voluntary muscle. The association of the striated muscle with response 

 to volition and, on the contrary, of the non-striated variety with involuntary action 

 must be accepted with certain reservations, since in some animals the development of 

 marked striation never takes place within the fibres of voluntary muscle. There is, 

 indeed, not a little evidence going to show that the structural differences which exist 

 between the striated and non-striated musculature are correlated with their physio- 

 logical activities, and that no fundamental distinction can be drawn between them 

 on purely morphological grounds. Muscles which in one group of animals possess 

 the characteristics of striated muscle-tissue may, in another group, be represented 

 by non-striated fibres (the muscles of the oesophagus, for instance), and it seems 

 probable that the greater portion of the voluntary cranial musculature is serially 

 equivalent to the involuntary musculature of the trunk. 



The non-striated or involuntary muscle represents a tissue less highly specialized 

 than the striped, the latter exhibiting to a conspicuous degree histological differentia- 

 tion. Constituting, in a way, a separate and intermediate group stands heart muscle, 

 which, while beyond the control of the will, presents striated fibres ; the latter occupy 

 histologically a place between the fibre-cell of the involuntary and the elongated 

 striated fibre of the voluntary muscle. It is desirable, therefore, to consider the sim- 

 pler type of contractile tissue before examining the more complex voluntary muscle. 



NON-STRIATED OR INVOLUNTARY MUSCLE. 



This, the less highly differentiated variety of muscular tissue, occurs in the form 

 of bundles and thin sheets principally within the walls of the organs and vessels, 

 although enjoying a wide distribution, seldom presenting robust masses, and being 

 entirely unconnected with the skeleton. Even when present in considerable amount, 

 this tissue is usually inconspicuous, presenting a faint yellowish tint. 



The distribution of non-striped muscle includes : i. The digestive tract, — the 

 muscularis mucosae from the oesophagus to the anus and delicate bundles within the 



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