ANIMAL MECHANICS. 



185 



made as to the distribution and arrangement of the fibres, 

 and 1 believe that the same arrangement exists in every 

 muscle, however complicated in form ; or, in other words, 

 that each muscular fibre passes from origin to insertion, re- 

 taining a constant thickness or cross section. 



In the common Heron, I obtained results quite conform- 

 able with those found from examination of the pectoral muscle 

 of the Buzzard. In Fig. 

 31 is shown the pectoral 

 muscle of the Heron 

 OP is the insertion on 

 the ridge of the arm 

 bone ; XYZis the curve 

 formed by the origin of 

 the muscular fibres, which 

 radiate in a fan-shaped 

 manner from their in- 

 sertion, OP, to their ori- 

 gin, XYZ. I measured 

 the length of the line 

 XYZ, and took the thick- 

 ness of the muscle at 

 various points along this 

 line ; and also along a 

 curved line of section, xyz, made so as to divide all the radi- 

 ating fibres. The cross sections along XYZ, and xyz are 

 shown in Fig. 32. 



The section XYZ had a mean thickness of 0.22 in. 

 through two-thirds of its length., and of 0.335 m * through 

 one-third, giving a mean thickness of 0.258 in. The section 

 xyz had a mean thickness of 0.56 in. in the middle third, and 

 of 0.50 in. and 0.38 in. in the outer thirds, giving a total 



X 



Fig. 31. 



