258 THE ANATOMY OF VERTEBRATED ANIMALS. 



The elevation of the wing is chiefly efl"ectecl by the pecto^ 

 rails 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 pecul- 

 iar 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 unit- 

 ing with the Jlexor digitorum perforatus. The result of this 

 arrangement is, that the toes are flexed whenever the leg is 

 bent upon the thigh, and, consequently, the roosting bird is 

 held fast upon his perch by the weight of his own body. 



In all the Sauropsida the cerebro-spinal axis is angulated 

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

 the latter being bent down toward the ventral side of the 

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

 posterior extremities originate may be enlarged in reptiles, as 

 in birds ; but, in the former, the posterior columns of the cord 

 remain parallel in the lumbar enlargement- while, in the lat- 

 ter, they diverge and give rise to the sinus rhomhoidalis^ 

 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 arach- 

 noid. 



The brain (Fig. 90) fills the cavity of the skull in the 

 higher Sauropsida^ and presents a well-developed cerebellum ; 

 a mesencephalon divided above into two optic lobes ; and 

 relatively large prosencephalic hemispheres, which attain a 

 considerable size in Crocodilia and A.ves, but never conceal 

 the optic lobes. In Crocodilia the cerebellum presents a dis- 

 tinct vermis, with transverse fissures. In birds the latter are 

 more distinct, and the lateral appendages of the cerebellum, 

 or flocculi^ become well defined, and are lodged, as in many 

 of the lower Maininalla^ in cavities of the side walls of the 

 skull, arched over by the anterior vertical semicircular 

 canal. 



There is no pons 'Varolii^ in the sense of transverse fibres 

 connecting the two halves of the cerebellum, visible upon the 

 ventral surface of the mesencephalon. The optic lobes con^ 

 tain ventricles. In lieptilia^ the optic lobes usually lie close 

 together upon the dorsal side of the mesencephalon, but in 

 Aves (Fig. 90 B, D) they are thrown down to the sides of the 



