PHYSIOLOGY. 



of a persoli speaking, move, and recol- 

 lect that an act of volition is necessary 

 for eacli motion. 



The distinguishing characteristic of 

 muscular fibres is their irritability, the 

 quality by which they contract in obedi- 

 eiw^e to the will, or on the application of 

 stimuli. This is an endowment residing 

 in all muscular organs, but not in equal 

 degree. The hollow muscles, which are 

 subservient to the vital functions, hold 

 the first rank ; these are followed by the 

 muscles of respiration, and the other vo- 

 luntary muscles close the enumeration. 

 It is doubted whether the arteries or the 

 large venous trunks be irritable. 



The contraction of a muscle consists fn 

 a shortening of its fibres, which are mark- 

 ed by transverse rugae, and feel indurat- 

 ed. But although its length is thus di- 

 minished, its circumference is propor- 

 tionally enlarged. These circumstances 

 produce an approximation of the move- 

 able points to which the muscle is attached, 

 and in this way all the motions of the bo- 

 dy are performed. 



An uninterrupted supply of blood, and 

 connection with the brain by the nerves, 

 is essential to the voluntary action of 

 muscles .- ligature of the arteries or 

 nerves destroys this power. But these 

 organs still retain the faculty of contract- 

 ing on the application of stimuli, even 

 after the connection with the brain be 

 cut off, and the animal be in other re- 

 spects dead : this power is the irritabili- 

 tas of Ilaller, the vis insita, or musctilaris ; 

 which, as that great physiologist and his 

 followers contend, is peculiar to the mus- 

 cular fibres exclusively. That this pro- 

 perty does not depend on the nerves is 

 clear, from the fact of several parts sup- 

 plied with nerves not possessing it ; and 

 from iis remaining after the nerves of a 

 part have been divided. 



The nerves may perhaps be regarded 

 as the more remote or exciting causes of 

 muscular motion, of which irritability is 

 the proximate or efficient cause. The 

 passions of the mind act on the sensori- 

 um, which reacts on the nerves of the 

 heart, and thus heightens the irritability 

 of that organ, exciting palpitation and 

 other irregular motions. The operations 

 of the will on our organs of motion may 

 be explained in the same way. 



This distinction of the causes of mus- 

 cular motion may be supported by the 

 experiments, in which the irritability 

 of the muscles has remained after para- 

 lysing a part, by tying or cutting its 

 nerves; and by cases of paralvsis. in 



VOL V. 



which sensation has remained in a limb 

 after its power of motion has ceased, or 

 rice versa. 



As it would be a fruitless labour to 

 enumerate and consider all the hypothe- 

 ses that have been framed concerning 

 muscular motion, we shall pass over that 

 part of the subject, and refer the reader 

 to the article GALVANISM for an account 

 of the effects of that principle on the mus- 

 cles. 



The real power of muscles is immense. 

 In the human body they are generally in- 

 serted near the centre of motion, and 

 consequently with a mechanical disad- 

 vantage ; so that much of their force is 

 expended in overcoming this obstacle. 

 Hence it has been calculated, that the 

 deltoid exerts a force equal to 2568 

 pounds to surmount a resistance of 50 

 pounds. The force with which a mus- 

 cle contracts is in a direct ratio with the 

 number of its fibres ; but the degree of 

 its contraction, and consequently the ex-i 

 tent of motions that it can effect on the 

 limb, is relative to the length of the fi.- 

 bres. The precise limits of contraction 

 in each fibre cannot be assigned ; for 

 though the long muscles of the extremi- 

 ties are supposed to diminish only a third 

 of their length in contraction, the circa- 

 lar fibres of the stomach, which, in the 

 state of extreme dilatation of this organ, 

 from circles of nearly a foot in diameter, 

 can contract to a ring of one inch in cir- 

 cumference. 



Our body contains about four hundred 

 and fifty muscles, which, when we consi- 

 der their wonderful and artificial co?;- 

 struction and collocation, and the united 

 advantages of firmness and mobility in 

 the instruments of motion to which they 

 are fixed, bestow on us two endowments 

 of the highest utility and consequence : 

 the greatest agility of the whole body and 

 of individual parts, combined with a won- 

 derful strength and power of enduring- 

 continued exertions. Both these prero- 

 gatives arise partly from the perfection 

 in the fabric of the muscles themselves : 

 which, as well as the perfect state of the 

 bones and joints, is most conspicuous 

 in the adult stage of life ; and partly 

 from exercise and habit, the influence 

 of which in augmenting the extent and 

 celerity of muscular motion is most con- 

 spicuous in the feats of the opera and 

 rope dancer, the runner, the boxer, the 

 jporter, &c. 



Voice and Speech. The voice is a sound 

 resulting from the vibrations which the 

 air suffers during its passage through the 



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