NEUROLOGY OF THE SATJKOPSIDA. 301 



the aponeurosis covering the ventral face of the hmgs, is 

 developed in all birds, but attains the greatest degree of 

 completeness in the Batitce, and especially in Apteryx. 



The muscles of the limbs are remarkably modified by the 

 excessive development of some of those found in other 

 Vertebrata, and the suppression of others. 



Thus in all birds possessing the power of flight, the 

 pedoralis major, as the chief agent of the downward stroke 

 of the wing, is very large and thick, taking its origin from 

 the whole length, and a great part of the depth, of the keel 

 of the sternum. 



The elevation of the wing is chiefly effected by the 

 pedoralis tertius, which arises beneath the foregoing muscle, 

 and passes over the inner side of the scapulocoracoid 

 articulation, as over a pulley, to reach the humerus. The 

 muscles of the forearm and digits are reduced, in accordance 

 with the peculiar modification of the skeleton of these parts. 

 In the hind limb of most birds there is a singular extensor 

 muscle, which arises from the pubis, ends in a tendon which 

 passes to the outer side of the knee-joint, and terminates 

 in the leg by uniting with the flexor digitorum perforatus. 

 The residt of this arrangement is, that the toes are flexed 

 whenever the leg is bent vipon the thigh, and, consequently, 

 the roosting bird is held fast upon his perch by the weight 

 of his own body. 



In all the Smu-opsida the cerebro-spinal axis is angulated 

 at the junction of the spinal cord with the medulla oblongata, 

 the latter being bent down towards the ventral side of the 

 body. The region in which the nerves of the anterior and pos- 

 terior extremities originate may be enlarged in reptiles, as 

 in bu-ds ; but, in the former, the posterior columns of the 

 cord remain parallel in the lumbar enlargement, while, in 

 the latter, they diverge and give rise to the sinus rhom- 

 boidalis, which is a sort of repetition of the fourth ventricle, 

 the dilated central canal of the spinal cord being covered 

 merely by a thin membrane consisting chiefly of the 

 ependyma and arachnoid. 



