610 



AVES 



[CH. 



bird's neck is a measure of the extent to which the pectoral girdle 

 has been pushed back. From the relation of the cervical spinal 

 nerves to the sympathetic ganglia it is certain that the greater part 

 of the length of the neck must be regarded as due to a secondary 

 zone of growth. From the middle of the sternum projects a great 

 vertical crest stretching outwards, called the carina or keel, and 

 it is from the sides of this mainly that the pectoral muscles take 

 their origin. There are two main muscles on each side.. First 

 the pectoralis major on the surface, which passes into a tendon 

 attached to the upper end of the humerus. The contraction of this 

 muscle brings about the down-stroke of the wing, the effective stroke 

 in flying. Underneath the pectoralis major is situated the pectoralis 

 minor, a much smaller muscle. Its tendon passes underneath 



the arch formed by the clavicle 

 and the coracoid bone, the latter of 

 which, as in Reptiles, connects the 

 shoulder-blade firmly with the ster- 

 num. Having passed through this 

 arch which is termed the foramen 

 triosseum because it is bounded by 

 three bones, viz. clavicle, coracoid and 

 scapula, the tendon is attached to 

 the back of the humerus, so that 

 the contraction of the muscle pulls 

 the humerus and thus the wing 

 upwards and backwards and not 

 downwards, the upper end of the 

 coracoid acting as a pulley round 

 which it passes. 



Returning to the wing, we must 

 now notice how the feathers are 

 arranged. The great quill feathers 

 are attached chiefly to the upper 

 and posterior edge of the hand, but 

 there are also a large number which 

 are implanted in the posterior surface of the ulna. These two groups 

 of feathers are pushed one over the other when the wing is folded, 

 just like the silk of a closed umbrella, but when the wing is stretched 

 out they only overlap very slightly, and thus a coherent and 

 practically air-tight surface is formed. Those feathers which are 

 attached to the hand are called primaries (6, Fig. 305, C), those 



FIG. 304. Shoulder-girdle and ster- 

 num of Peacock, Pavo cristatus 

 xf. 



1. Carina of the sternum. 2. Cora- 

 coid. 3. Scapula. 4. Clavicle. 

 5. Costal process. 6. Surfaces 

 for articulation with the sternal 

 ribs. 7. Posterior (xiphoid) 

 and oblique processes. 



