178 



ANIMAL MECHANICS. 



meet in a central raphe. 118, which is the direction of the re- 

 sultant motion produced by the simultaneous contraction of 

 all the penniform fibres. The lengths of the fibres measured 

 from behind forwards increase from 0.95 in. to 1.50 in., and 

 then decrease gradually down to zero. The angle 0, made by 

 them with the central raphe, is 45 0 , when the top of the 

 tongue bone is brought into the same plane as the floor of the 

 mouth. If / denote the length of any fibre, and SI the amount 

 of its contraction, it follows that any point x, lying in the 

 central raphe, will be drawn upwards and forwards through 

 a space represented by Sl\/ 2, where I is the length of the 

 line xa, which is proportional to the distance of x from the 

 symphysis of the jaw ; hence the point S will have no motion 

 at all, and the velocity will increase from S to the centre of 

 //, where the fibres are of the greatest length. 



Another example of penniform muscle may be seen in the 

 accelerator urince, in which the fibres form angles of 23 0 

 with the central raphe. 



Muscular Fibres rectilinear, lying in the same Plane, and not 

 parallel to each other. 

 This natural group of muscles may be divided, like the 

 former, into 

 three distinct 

 types, which 

 may be nam- 

 ed, respec- 

 tively, Tri- 

 angular, Del- 

 toiclal, and 

 Quadrilateral 

 muscles. 



Triangular Muscles. — In these muscles, the origin is a 



Fig. 28. 



