XIII 



PHYLUM CHORD ATA 



391 



angle. The glenoid cavity (gl. cv.) is formed in equal proportion by 

 the two bones ; internal to it the scapula is produced into an acromion 

 process. 



In front of the coracoids is a slender V-shaped bone, thefurcula 

 (fur.) or " merrythought," the apex of which nearly reaches the 

 sternum, while each of its extremities is attached by ligament to 

 the acromion and acro-coracoid 

 processes of the corresponding 

 side in such a way that a large 

 aperture, the foramen triosseum 

 (f. trs.) t is left between the 

 three bones of the shoulder- 

 girdle. The furcula is an in- 

 vesting bone and represents 

 fused clavicles and inter- 

 clavicle. 



Equally characteristic is the 

 skeleton of the fore-limb. The 

 humerus (Fig. 1034, hu.} is a 

 large, strong bone, with a 

 greatly expanded head and a 

 prominent ridge for the in- 

 sertion of the pectoral muscle. 

 In it, as in all the other long 

 bones, the extremities as well 

 as the shaft are formed of 

 true bone. The radius (ra.) is 

 slender and nearly straight, 

 the utna stouter and gently 

 curved. There are two large 

 free carpals, a radiate (ra!) 

 and an ulna-re (ul. 1 ), and articu- 

 lating with these is a bone 

 called the carpo-metacarpus 

 (cp.nitcp.) consisting of two 

 rods, that on the pre-axial side 

 strong and nearly straight, 

 that on the postaxial side 

 slender and curved, fused 

 with one another at both their 

 proximal and distal ends ; the 



proximal end is produced, pre-axially, into an outstanding step-like 

 process. The study of development shows that this bone is formed 

 by the union of the distal carpals with three metacarpals (Fig. 

 1035), the second and third of which are the two rod-like portions 

 of the bone, the first the step-like projection. Articulating with 

 the first metacarpal is a single pointed phalanx (ph. 1) ; the second 



B B 2 



FIG. 1034. Columba livia. Skeleton of the 

 left wing, cp.mtcp. carpo-metacarpus ; hu. 

 ; humerus ; ph. 1, phalanx of first digit ; ph. 2', 

 ph. f", phalanges of second digit ; ph.3, phalanx 

 of third digit ; pn.for. pneumatic foramen. 

 ra. radius ; ra'. radiale ; ul. ulna ; ul'. ulnare. 



