FUNCTIONS OF MUSCULAR TISSUE. 317 



power of the nerves ; for, when the latter have recovered their capacity, the 

 muscles refuse obedience to their stimulation, and can only be brought to act 

 by persevering and judiciously contrived exercise. But notwithstanding the 

 energy of nutrition in Muscular tissue, the rapid interstitial change which 

 takes place in it when actively exercised, and the complete restoration of its 

 normal constitution after degeneration from disuse, it is doubtful if any true re- 

 generation ever takes place in it, when there has been actual loss of substance. 

 Wounds of Muscles are united by Areolar tissue, which gradually becomes con- 

 densed ; but its fibres never require any degree of contractility. 



314. The property of Contractility on the application of a stimulus, appears 

 to be limited, in the fully-developed Human organism, to the two forms of Mus- 

 cular tissue which have been now described ; several tissues which exhibit it, and 

 which yet do not present any obvious evidence of muscular structure, having 

 been shown by Prof. Kolliker to contain the fusiform cells which constitute 

 the non-striated fibre. It is characteristic of Animal contractility, as distin- 

 guished from that which is concerned in producing the sensible movements of 

 Plants, that whilst it is capable of being called into play by stimuli of various 

 kinds (mechanical, chemical, electrical, &c.), which also act upon the Vegetable 

 contractile tissues, yet it is excited in addition by the stimulus of Innervation, 

 that is, by the operation of Nervous force, to which the tissues of Plants are 

 not amenable. 1 And it is when its peculiar property is thus made to display 

 itself, that the Muscular tissue becomes the instrument of the operation of the 

 Nervous system upon the external world, and thus performs an important part 

 in the purely Animal Functions. The Muscular tissue, however, is not always 

 thus called into activity through the medium of the Nervous system; for it is 

 employed to execute numerous movements, which are immediately connected 

 with the maintenance of the Organic functions, and in which the influence of 

 Innervation seems to be but little concerned ; its contractility being excited to 

 action by stimuli directly applied to itself. The two forms of Muscular tissue, 

 the striated and the non-striated, are for the most part appropriated to these 

 two purposes respectively ; the former being the kind most readily acted on 

 through the Nervous system, and being invariably employed in the Muscles 

 that are ordinarily called into action by its influence ; whilst the latter is with 

 difficulty excited to contraction through the Nervous System, and is usually 

 employed in Muscles whose action is altogether uncontrollable by the will. 



315. This general property of Contractility shows itself under two forms, which 

 are alike distinct in the mode of their action, and in the conditions requisite for 

 its excitation. Its most obvious and striking manifestations present themselves 

 in the Voluntary muscles and in the Heart; which, when in activity, exhibit 

 powerful contractions tending to alternate with relaxations. The modification 

 of contractility which is concerned in producing these, is distinguished as Irri- 

 tability. On the other hand, we find that the muscles exhibit a tendency to a 

 moderate and permanent contraction, which is not shown by them when they 

 are dead, and which cannot, therefore, be the result of elasticity, or of any 

 simple physical property ; and this contraction, instead of being consequent 

 upon stimulation through the nerves, is especially excited by changes of tem- 

 perature in the tissue itself. This endowment, which seems to exist in the 

 greatest amount in certain forms of the non-striated muscle is called Tonicity. 

 These two modifications of Muscular Contractility require a separate consi- 

 deration. 



316. Of Muscular Irritability. All Muscular Fibres which are in possession 

 of vital activity, may be caused to contract by stimuli directly applied to them- 

 selves ; and these stimuli may be of different kinds. The simplest is the con- 



1 See "Princ. of Phys., Gen. and Comp.," CHAP xix., Am. Ed. 



