102 THE MECHANICAL FUNCTIONS. 



sue; which thus forms a separate investment for each. This 

 plan of successive reunion into larger and larger assemblages 

 is carried on through several gradations of size, till the en- 

 tire muscle is completed. 



That we may be the better able to appreciate the excel- 

 lence of the plans adopted in the mechanism of the animal 

 frame, let us inquire what arrangements would occur to us, 

 prior to an acquaintance with those actually adopted, as the 

 most advantageous dispositions of the muscular power. It 

 is evident, that the simplest mode would be that of extend- 

 ing the fibres of the muscle in a straight line between the 

 points intended to be brought nearer to each other. This 

 direct application of the power, however, is seldom compa- 

 tible with convenience, unless the parts to be moved are of 

 very small size, and require very delicate adjustments. 

 Straight muscles, accordingly, are employed chiefly for the 

 movements of the minuter parts of the apparatus belonging 

 to the senses, such as the eye, and the ear, and also that of 

 the voice. In insects, when the hard case, or skeleton, is 

 wholly external, this direct application of the moving force 

 is also very generally employed. The shells of the bivalve 

 mollusca, as of the Oyster and the Cardhim, are closed by 

 one or two straight muscles, the fibres of which pass imme- 

 diately from the inner surface of the one to that of the other. 



In the greater number of cases it is more convenient to 

 place the muscle in a situation which causes it to act ob- 

 liquely with respect to the direction of the motion produced 

 in the part to which it is attached. This will, of course, be 

 attended with a loss of force corresponding to the degree of 

 obliquity; but there are, at the same time, advantages gained, 

 both in point of velocity of motion, and also in the effect 

 being produced by a smaller extent of contraction in the 

 fibres of the muscle. Oblique muscles are frequently em- 

 ployed in pairs, and are made to act on opposite sides of the 

 line of the intended motion, which is, in this case, the dia- 

 gonal between the direction of the two equal forces. Thus, 

 in order to bring a bone at p. Fig. 39, down to the point q, 

 the two muscles a and b, extending from the fixed points 



