PHENOMENA OF MUSCULAR CONTRACTION. 401 



action. This disposition of antagonist muscles prevails in almost every 

 part of the frame, and will require notice presently. 



Muscles are not, however, the sole agents in replacing parts. Many 

 elastic textures exist, which, when put upon the stretch by muscular 

 contraction, have a tendency to return to their former condition, as 

 soon as the extending cause is removed. Of this a good example 

 occurs in the cartilages of prolongation, which unite the ribs to the 

 sternum. During inspiration, these elastic bodies are extended; and, 

 by returning upon themselves, they become active agents of expiration, 

 and tend to restore the chest to its unexpanded state. 



The production of the phenomena of muscular contraction is, so far 

 as is known, unlike any physical process with which we are acquainted. 

 It has, therefore, been considered essentially organic and vital ; and, 

 like other operations of the kind, will probably ever elude our researches. 

 Yet here, as on every obscure subject, hypotheses have been innumera- 

 ble; varying according to the fashionable systems of the day, or the 

 views of the propounder. They, who formerly believed that the mus- 

 cular fibre is hollow, or vesicular, ascribed its contraction to distension 

 by the influx of "animal spirits" or of blood; and relaxation to the 

 withdrawal of those. Such were the hypotheses of Borelli, 1 Stuart, 2 

 and others. Independently, however, of the objection to these views, 

 that we have no positive evidence of the existence of such vesicles, it 

 is obvious, that the explanation is defective, inasmuch as we have still to 

 look to the cause that produces this mechanical influence. Again, 

 how are we to account, under this hypothesis, for the surprising efforts 

 of strength executed by muscles? The mechanical influence of animal 

 or other spirits granting for a moment their existence might develope 

 a certain degree of force ; but how can we conceive them able, as in the 

 case of the muscles inserted into the foot, to develope such a force as 

 to project the body from the ground? In all these cases, a new force 

 is generated in the brain; and this, by acting on the muscular fibre, is 

 the efficient cause of the contraction. Moreover, what an inconceivable 

 amount of fluids would be necessary to produce the contraction of the 

 various muscles, that are constantly in action; and what, it has been 

 asked, becomes of these fluids when relaxation succeeds to contraction? 

 Some have affirmed, that they are absorbed by the venous radicles; 

 others, that they run off by the tendons; and others, again, that they 

 become neutralized in the muscle, and communicate to it the greater 

 size it attains under exercise. These fantasies are too abortive to 

 require comment. 



When chemical hypotheses were in fashion in medicine, physiology 

 participated in them largely. At one- time it was imagined, that an 

 effervescence was excited in the muscle by the union of two substances, 

 one of which was of an acid, the other of an alkaline nature. Willis, 

 Mayow, Keill, 3 Bellini, 4 &c., supported opinions of this kind; some 

 ascribing the effervescence to a union of the nervous fluid with the arte- 



1 De Motu Animalium, Lugd. Bat., 1710. 

 3 De Structura et Motu Musculari, Lond., 1738. 



3 Tentamin. Medico-Physic., No. v., Lond., 1718. * Bostock, op. cit., p. 111. 



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