319 



wing; in the former, it is white, tender and delicate, while, in the latter, 

 it is tough, stringy, dark-coloured, carbonaceous, and of a very strong- 

 smell. Respiration, of which the principal use is to impregnate arterial 

 blood* with the oxygen necessary to the contractions of the muscular 

 fibre, is more complete, decomposes the greater quantity of atmospheri- 

 cal air, in those animals that are naturally destined to most exertion. 

 Those birds which support themselves in the air by powerful and fre- 

 quent motions, have likewise, the most active respiration. Athletes, who 

 astonish us by the developement of their muscular organs, and by the 

 powerful efforts of which they are capable, all have a very ample chest, 

 a powerful voice, and very capacious lungsf. In running, as there is a 

 considerable consumption of the principle of motion, we pant, that is, 

 we breathe in a hurried manner, that there may be the. greatest possible 

 quantity of blood oxydized to perform the contractions necessary to the 

 exercise of running. 



CLXV. Of the preponderance of the flexors over the extensors\. The ex- 

 tensor muscles are, generally, weaker than the flexors; hence the most 

 natural position, that in which all the powers are naturally in equilibrio, 

 that which our limbs assume during sleep, when the will ceases to deter- 

 mine the vital influx to the parts under its controul, that in which we can 

 continue longest without fatigue, is a medium between flexion and exten- 

 sion, a real state of semi-flexion. 



Attempts have been made to discover the cause of this preponderance 

 of the flexor muscles over their antagonists. According to Borelli, the 

 flexors being shorter than the extensors of the same articulation, and con- 

 tracting equally^, the former must occasion a more extensive motion of 

 the limbs, and determine them towards a state of flexion. But it is, in 

 the first place, incorrect to say, that the flexors are shorter than the ex- 

 tensors; and, in the next place, if we are to estimate, by the length of a 

 muscle, the extent of motion that may be produced by its action, we ought 

 not to measure the whole of the fleshy part, nor to include in the calcula- 

 tion, the tendinous cord which terminates it, but UTconsider the length 

 of its fibres, on which depends entirely the extent of motion produced by 

 its contractions. 



The degree of decurtation of which a muscle is capable, is always pro- 

 portioned to the length of its fleshy fibres, as is the power of contraction 

 to the number of the fibres. Now, if the fibres of the flexors are in greater 

 number than those of the extensors, it follows as a necessary conse- 

 quence, that the limbs will be brought into a state of flexion, when 

 the principle of motion shall be distributed to them in an equal quantity^ 

 and even though the number of fibres should be the same in the flex- 



* See APPENDIX, Note W. 



f I never saw a very strong 1 man that had not broad shoulders, which indicates a con- 

 siderable developement of the cavity of respiration. If there be individuals that seem 

 to be exceptions to this general law, it is that by frequent exercise, and by a laborious 

 life, they have increased the natural power of their muscles. This increase is seldom 

 universal, but almost always limited to certain parts which have been most employed ; 

 as the arms, the legs, or the shoulders. Author's Note. 



t The theory of the preponderance of the flexors is entirely my own, and was first 

 proposed by me, in the collection of Memoirs of the Medical Society of Paris, for the 

 year VII. of the Republic (1799.) Author's JVote. 



Musculi flexores ejusdem articuli breviores sunt extensoribus, et utrique jcque con- 

 trabuntur Prop. 180, de motu animalium. 



