CONTRACTILITY. 



719 



muscles provided with sensitive nerves, by 

 which the mind is informed of the contractions, 

 and so enabled to regulate or guide them. The 

 Sensations act chiefly on this description of 

 muscles likewise, but partly also on muscles 

 the nerves of which give no such distinct inti- 

 mation of their contractions, and which are 

 uniformly and strictly involuntary; and the 

 chief excitants of this last class of muscles in 

 the Animal Economy are physical stimuli, ap- 

 plied to themselves and to their lining mem- 

 branes. 



II. In regard to the vital power or property 

 of Irritability, as exhibited in any of these ways, 

 the following facts demand particular notice. 



1. The minutest fibres, of which the mus- 

 cles, exhibiting this property, consist, or what 

 have been called by some authors the primary 



filaments of muscular fibres, appear under the 

 microscope to consist of rows of globules, or 

 at least to be marked by transverse striae, at 

 equal distances. 



2. When contraction takes place, these fila- 

 ments, or rows of globules, are thrown into a 

 zigzag form ; the angles being always at the 

 same points on each contraction, and being 

 generally obtuse, rarely and only on occasion 

 of very forcible contraction, acute.* At the 

 points where these angles are formed, the fila- 

 ments are crossed, according to the observa- 

 tion of Prevost and Dumas, by nervous fibrils; 

 but it is important to remember, that this last 

 observation has been made only on muscles 

 of voluntary motion, and on them only in 

 cold-blooded animals, where they are somewhat 

 translucent. 



3. According to the best observations, not 

 made on entire limbs or even entire muscles, 

 which involve various fallacies, but on small 

 portions of muscles, removed from living 

 bodies, it appears that no alteration of the 

 bulk of the filaments attends this alteration of 

 their form, so that neither the size nor distance 

 of the particles or globules appears to be 

 changed, but merely their position in regard 

 to one another .f 



4. When by any stimulus, applied to mus- 

 cular fibres, the filaments directly stimulated 

 are thrown into action, the contractions very 

 generally and rapidly extend to many others 

 in their neighbourhood, frequently even to the 

 whole muscle of which they form a part ; but 

 the contractions of single fibres appear to be 

 of short duration, and the more enduring 

 efforts to be made by many short successive 

 contractions and relaxations or vibrations of 

 the individual fibres. 



From these facts it would appear, that each 

 exertion of this property of Irritability essen- 

 tially consists in a greatly increased attraction 

 among the particles or globules constituting 



* This appearance, with slight variations, has 

 been repeatedly seen, both in warm-blooded and 

 cold-blooded animals, in the lower classes and 

 even in the infusory animals, under the micro- 

 scope. 



t See Prevost and Dumas, in Journal de Phy- 

 siologic, torn. iii. j and Mayo's Outlines. 





the muscular fibres, and alteration of the di- 

 rection in which this attraction acts, rapidly 

 communicated from one particle to another, 

 both along the same fibre, and among adjacent 

 fibres, and rapidly succeeded by repulsion,, 

 or return to the previous state of the cohesive 

 attraction existing among these particles. 



When a muscular mass, consisting of many 

 such fibres, is thrown into this kind of action, 

 it is easy to understand that its breadth and 

 thickness, and its rigidity or resistance to com- 

 pression will be increased ; that its extremities 

 will be approximated ; and that if it be dis- 

 posed around a cavity, containing a fluid and 

 provided with an outlet, that fluid will be 

 expelled. It is by such contractions, that all 

 the more conspicuous movements, even of the 

 organic life, of the higher animals, are per- 

 formed, and that their locomotive and vocal 

 powers are exerted ; and it is worth while to 

 pause for a moment to consider the almost in- 

 conceivable amount of moving power, and ra- 

 pidity of motion, which various facts indicate 

 in muscles thus contracting, whether under the 

 influence of the will, or from other stimuli. 



It seems well ascertained that the contrac- 

 tions of the left ventricle of the human heart, 

 in its ordinary unexcited state, are sufficient to 

 expel its fluid contents, in free space, a dis- 

 tance of 7 feet, and to balance a weight of 

 above 50 Ibs. ; and this power is exerted regu- 

 larly more than once in every second, and 

 often, even independently of disease, twice in 

 every second, during the whole of human life. 

 The ordinary action of the left ventricle of the 

 Whale's heart suffices to expel, according to 

 Dr. Hunter's statement, at each pulsation, 

 above ten gallons of blood, with a great velo- 

 city, through a tube of a foot in diameter. 



Two instances given by Haller, and quite 

 within the limits of ordinary experience, suf- 

 ficiently exemplify the great power occasion- 

 ally exerted by voluntary muscles ; and which 

 will appear the more extraordinary when it is 

 remembered, that the direction of muscular 

 fibres, as regards the line in which they are to 

 act, the points of their insertion into the 

 bones they are to move, and the line in 

 which they act, as regards the motion which 

 they give to these bones, are all, very ge- 

 nerally, such as to render. their action dis- 

 advantageous, and require a greater amount 

 of moving power than might otherwise have 

 been necessary. The instances recorded are, 

 the case of a man, who could raise a weight 

 of 300 Ibs. by the action of the elevator mus- 

 cles of his jaw; and that of a slender girl, 

 afflicted with tetanic spasm, in whom the ex- 

 tensor muscles of the back, in the state of 

 tonic contraction or opisthotonos, resisted a 

 weight of above 800 Ibs., laid on the abdomen 

 with the absurd intention of straightening the 

 body. In some of the lowest classes of ani- 

 mals the intensity of the muscular power ap- 

 pears to be greater than in any of the largest. 

 Thus, a flea has been known to leap sixty times 

 its own length, and to move as many times 

 its own weight. 



