CONTRACTILITY. 



717 



or of the stiffening of limbs after death,) he 

 gives this name to contractions which are 

 strictly vital. Almost all animal substances 

 are liable to contraction from heat, and from 

 the application of various chemical agents 

 which affect them as astringents, to which 

 property Bichat gave the name of Contractilite 

 par racornissement ; and it is easy to perceive 

 that this property also, although persistent in 

 the perfectly dead body, and therefore inde- 

 pendent of life, may give occasion to contrac- 

 tions which may sometimes be mistaken for 

 indications of the strictly vital contractility. 



Confining our attention, however, to such 

 contractions of the solids of organized bodies, 

 as are exhibited by them only in their living 

 state, i.e. so long as they present that assem- 

 blage of phenomena, to which we give the 

 name of Life, we proceed to state the facts 

 which seem to be most important and best 

 ascertained, first, as to the modes in which 

 they are excited ; secondly, as to their pheno- 

 mena, and varieties ; thirdly, as to the condi- 

 tions necessary to their manifestation ; and, 

 lastly, as to the laws which regulate them. 



I. It is universally known, that the most 

 striking examples of vital contractions are seen 

 in the effects produced by various stimuli 

 acting on muscles, particularly those of volun- 

 tary motion, and the heart. The essential cha- 

 racters of muscular fibres, their composition 

 nearly akin to the fibrin of the blood, their 

 arrangement in parallel fasciculi, which are 

 bound together by cellular membrane, their 

 soft texture, and slight elasticity are also ge- 

 nerally known. The change excited by sti- 

 muli acting on them is a contraction in the 

 direction of the visible fibres of the muscle, 

 which in the healthy state always rapidly 

 alternates with relaxation; and by these two 

 circumstances, the excitation by stimulus, 

 and the quickly ensuing relaxation, we dis- 

 tinguish that form of Vital Contractility, to 

 which the term Irritability is most correctly 

 applied. 



The stimuli which produce this effect are 

 very various; and the experience of our own 

 bodies points out the obvious distinction of 

 these into physical and mental. Of the first 

 kind, air and water, especially if aided by heat, 

 act decidedly in this way; but those which 

 have been chiefly used in experiments are, dis- 

 tension, especially in the case of the hollow 

 muscles, such as the heart or bladder, che- 

 mical acrids, such as acids, alkalies, various 

 alkaline, earthy, or metallic salts, and elec- 

 tricity or galvanism. The effect of all these 

 stimuli is much increased by their being sud- 

 denly applied. 



It has also been long known, that many 

 muscles are excited to contraction by such sti- 

 muli, when applied to certain nerves, entering 

 their substance, or to certain parts of the spinal 

 cord or brain, even more effectually than by 

 applications to themselves; and likewise, that 

 it is only when those nerves are entire, up to 

 the brain, that those muscles which are natu- 

 rally obedient to the mental stimulus of the 

 Will, can be excited by voluntary efforts. 



From these different modes of excitation of 

 the contractile power of muscular parts, diffe- 

 rent names have been given to the power 

 itself, as by Haller, who applied the term 

 Vis Tonica to the contraction from distension, 

 Vis Insita to the contraction from irritation of 

 the muscular fibres themselves, Vis Nervosa to 

 the contraction from irritation of a nerve, and 

 Vis AnirnaUs to the contraction from volition, 

 acting at the brain and transmitted through a 

 nerve; or again by Bichat, who applied the 

 term Contractiliti Qrganique Sensible to the 

 contractions excited by any kind of irritation, 

 acting on muscular fibres themselves, and the 

 term Contract ilitt Animate to those excited by 

 stimuli, whether mental or physical, acting on 

 the nerves, spinal cord, or brain. But it is 

 obviously more correct to distinguish the dif- 

 ferent varieties of the vital power according to 

 the phenomena, which the contracting part 

 presents, than according to the manner in 

 which the contractions are excited ; and there- 

 fore those terms have fallen much into disuse. 

 In most instances, it is the same vital power of 

 Irritability, as above defined, which is called 

 into action in these different ways. 



It is only of late years, that it has been 

 fully ascertained, as to the excitement of vital 

 contractions through nerves : 1, that it is, almost 

 exclusively, in the case of muscles which are 

 naturally subject to the Will, that even physical 

 irritation, confined to the nerves, has power 

 to excite contraction; and 2, that these muscles 

 have nerves, or nervous filaments, from two 

 distinct sources, viz. from the anterior and 

 posterior columns of the spinal cord, and their 

 prolongations within the cranium ; and that 

 it is by irritation of the first of these only, 

 (or almost exclusively,) that the muscular con- 

 tractions are excited.* From these facts, it 

 appears obvious, that the grand and eternal 

 law of separation, as Haller calls it, of the Vo- 

 luntary and Involuntary muscles, consists essen- 

 tially, not in different powers of the muscular 

 parts, but in different endowments of the ner- 

 vous filaments which enter them. 



In regard to the excitation of muscular con- 

 traction through nerves, it is also to be ob- 

 served, that although the action of muscles in 

 obedience to the will is the most obvious and 

 striking example, in the living body, where 

 the intervention of a change in a nerve is known 

 to be an essential condition of the act, yet 

 there are many examples of movements, per- 

 formed by voluntary muscles, in obedience to 

 mental stimuli, but not to volitions, to sensa- 

 tions, or other involuntary acts of mind, even 

 in opposition to efforts of the will. These con- 

 stitute a very important class of vital motions, 

 and are known to be equally excited through 

 the motor nerves of the muscles concerned in 

 them. Of this kind are not only the irregular 

 agitations of the limbs produced by tickling, or 

 the convulsive writhing of the body from pain, 

 but also, such regular and admirably precise 

 movements as shrinking when pain is excited 



* Sec Mayo's Outlines, 2d edit. \>. 50 et scq. 

 ;md Sir C. Bell, Phil. Trans. 1826. 



