DEVELOPMENT OF THE MUSCLES. . 421 



a way as to act at a mechanical disadvantage (?'. e. so as to act at 

 first nearly parallel to the surface of bones they move, or near the 

 extremities of the latter). Thus their effective is much less than 

 their intrinsic force. E. g. the intrinsic force of the deltoid has 

 been estimated at 1,000 pounds; while its available force is perhaps 

 not more than 50 pounds. As a compensation for this loss of force, 

 greater velocity and extent of motion of the part are secured with 

 a given amount of shortening of the muscular fibres. 



The tonicity of the muscles has been spoken of on page 403. 



The muscles possess sensibility, though of a peculiar kind; they 

 becoming painful and sensitive to pressure after long-continued ac- 

 tion, and after being affected with cramps or spasms ; while scarcely 

 any sensation is excited by punctures, burns, and incisions into their 

 substance. They also possess a delicate sense of feeling for their 

 own state of contraction, and estimate very minute variations in the 

 force with which they act. It is this kind of sensibility (called the 

 muscular sense) which enables us to judge of the weight of objects. 

 The sense of fatigue is also peculiar to the muscles. It has already 

 been shown that the muscles contain but few sensitive nerve-fibres 

 (p. 415) enough, probably, to give merely the slight impressions 

 normally characterizing the muscles in ordinary circumstances; 

 while they may also give rise to pain, even, when compressed for a 

 time during their contraction. 



The muscles, during their contraction, elicit a peculiar sound, of 

 a silvery character, and which somewhat resembles the rumbling of 

 distant carriage- wheels. This is probably produced by modifica- 

 tions in the circulation, as well as by the changes occurring in the 

 fibres themselves. 



Heat is also developed by muscles during contraction, as has 

 already been explained (p. 402); oxygen being absorbed and car- 

 bonic acid being given off by them at the same time. 



Development of the Muscles. 



The manner in which the striated muscular fibres are developed, 

 has already been explained (p. 399). The muscles are not evident 

 as distinct organs in the human foetus before the end of the second 

 month, and can then be seen only under the microscope ; and the 

 tendons cannot be distinguished from the other portions. In the 

 tenth to the twelfth week they are more distinct ; and the tendons 



