318 



done by Steno in the same situation*. And this is a further proof of what 

 we have said elsewhere, of the stupifying qualities of the blood which 

 flows in the veins. 



The irritability of the muscles destined to voluntary motions is propor- 

 tioned to the size and number of the nerves and arteries which are 

 distributed to their tissuef. The tongue, which of all the contractile 

 organs receives the greatest number of cerebral nerves, is, likewise, that 

 which, of all those under the controul of the will, has most extent, most 

 freedom, and most variety of motions!. The muscles of the larynx, and 

 the intercostals, receive nearly as many, considering the smallness of 

 these parts. 



CLXIV. Of all the hypothesis applied to the explanation of the phe- 

 nomena of muscular contraction, that appears to me the most ingenious 

 and the most probable, which makes it to depend on the combinations 

 of hydrogen, of carbon, of azote, and other combustible substances in 

 the fleshy part of the muscle, with the oxygen conveyed with the blood 

 by the arteries. 



To effect this combination, it is necessary, not only that the muscle be 

 supplied with arterial blood, and that oxygen come in contact with the 

 substances which it is tooxydize, but it is required, that a stream of nerv- 

 ous fluid should penetrate through the tissue of the muscle, and determine 

 the decompositions which take place, as the electrical spark gives rise to 

 formation of water by the combination of the two gases of which it con- 

 sists. According to this theory, first proposed by Girtanner, all the 

 changes which take place, during the contraction of a muscle, theturges- 

 cence, the decurtation and the induration of its tissue, its change of tem- 

 perature depend on this reciprocal action of the elements of the muscular 

 fibre, and of the oxygen of arterial blood. 



Muscular flesh is harder, firmer, and more oxydized, according as the 

 animaltakes much exercise. We well know, what a difference there is, 

 between the flesh of wild and of the domestic animals ; between the flesh 

 of our common fowl and that of birds accustomed to remain long on the 



* Ligature of the aorta may be conceived to produce paralysis, owing chiefly to the 

 stop it puts to the circulation of arterial blood in that part of the spinal cord, and in the 

 neurilema of the nerves, below where the ligature is placed. 



I See APPENDIX, Note F F. 



$ It is scarcely necessary to repeat, that I am not speaking of those motions, more or 

 less involuntary, performed by muscles which receive tiie nerves, in part or wholly, 

 from the great sympathetics. Though the particular nature of these nerves has a re- 

 markable influence on the organs to which they are distributed, we find that the general 

 rule is almost without exception, for the heart and diaphragm which hold the first rank 

 among the parts endowed with irratibility, receive a considerable number of vessels and 

 nerves. Authur's Note. 



The disposition of the muscles to contract is different from, and even opposite to, 

 their energy of contraction. The feeble muscles of an hysterical female contract so 

 readily and" so frequently from the slightest irritation, as nearly to appear involuntary 

 in their actions ; while, on the contrary, the powerful muscles of the athlet act only 

 from energetic stimuli and from fully expressed volition. Thus we observe in feeble in- 

 dividuals a certain mobility of muscular parts and organs, which does not exist in the 

 robust, as if the irritability of such parts were excited with a facility in proportion to 

 the deficiency of energetic action. The disposition of muscles to contract differs also 

 according to the age of the animal, and it bears even some relation to organization of 

 the muscle itself. Copland. 



See APPENDIX, Note F F, for remarks on the period at which the voluntary muscles 

 are formed, find on their appearances and constitution, at the different periods of life. 



