ANIMAL MECHANICS. 



spines of the scapulae are shown at AA and at BE ; the line 

 AB joining the vertices of the two triangular muscles is 

 necessarily perpendicular to the spinal column IF; and the 

 bisectors of the vertical angles (AO and BO) are found to fall 

 behind and below the line AB ; so that when the whole 

 trapezius is in action, the resultant forces draw the scapulas 

 in the directions AO and BO, shown by the arrows. This 

 action of the trapezius muscle is usefully employed, for ex- 

 panding the chest, in the kalisthenic exercises used by young 

 ladies. 



Using the same notation as in the case of the biceps fe- 

 moris muscle of the lion, I have found the following results 

 from measurements made on the trapezius muscle of man : 



XY = 1 6 inches. 

 XA = 9 „ 

 YA = 12 „ 



jS = 8 3 ° = A OX. 

 0 = 47° 3 o' = OAX. 

 p = 7 in. 

 q = $ : 16 in. 



From these data we find, from equations (30) and (31), 



Work done 2q 



Inherent work ~" ^ cot f (j3 - 9) 



10.32 P cot a (/3 + 9) 



1 I cot. 170 45' \ 

 V cot * 6 5 15 I 



The work done by the trapezius muscle is, therefore, only 

 77 per cent, of its inherent work, and there is a loss of 23 per 

 cent, due to the fan-shaped arrangement of its fibres. 



The pectoral muscles of many animals, especially the 

 rodents, form, by union of the muscles of the right and left 

 sides, admirable examples of Deltoidal muscles. 



In fig. 38 I have drawn the pectoral muscles of the com- 

 mon squirrel, 0, 0 being the humeral insertions of the two 

 0 



