62 



ANIMAL MECHANISM. 



rigorously to the general laws which we have just enunciated. 

 The larger a muscle is, that is to say, the more extensive is 

 its surface, the more susceptible it is of considerable eJSbrt. 

 But, on the other hand, a muscle contracts only in proportion 

 to its own length. We may estimate that the mean shortening 

 of a muscle while contracting, when it is not detached from 

 the animal, is about a third of its length when in repose. It 

 follows that the work done by a muscle will be in proportion 

 to its length and its transverse section ; that is to say, to its 

 volume or to its weight. 



Thus, it is possible to ascertain, according to the anatomi- 

 cal characters of a muscle, what is the force which it pos- 

 sesses, relatively to that of other muscles of the same animal, 

 and what is the form under which its work is done. 



The substance of the muscles, that is to say, of red flesh, 

 presents the same density in the diJfferent parts of the animal 

 frame ; in consequence of which the weight is the most exact 

 and the most expeditious method of estimating the relative 

 importance of two masses of muscle, and of predicting the 

 quantity of work which they are able to execute. 



As to the form under which muscular work must be pro- 

 duced, it is deduced not less easily from the form of the 

 muscle. If it be thick and short, it should produce a strong 

 effect multiplied by a short range ; if it be long and slender 

 it will have a more extended range, but will only develop 

 feeble energy. 



There are many examples in proof of this law which 

 regulates muscular action — the sterno-mastoidal, the sarto- 

 rius, and the rectus abdominis, are muscles of a long range, 

 or, as it may be otherwise expressed, having a great ex- 

 tent of movement ; they have a fleshy portion of greater 

 length. The large pectoral muscle, the gluteus maximus, 

 or the temporal muscle are large and short muscles, that 

 is to say, capable of a considerable effort, but of slight 

 contraction. 



Borelli already understood the laws of muscular force ; 

 without the intervention of the notion of work , which was not 

 introduced into mechanics at the time when he lived; he 

 made a very clear distinction between these two opposite 



