238 NERVOUS SYSTEM. 



vertebrata. The choroid plexus is obvious in each of the 

 lateral ventricles, which communicate below with the third 

 ventricle, and the corpora striata form lengthened transverse 

 eminences. The third ventricle communicates behind with 

 the fourth by the aqueductus Sylvii beneath the transverse 

 valve of Vieussenius, and is continued downwards in front 

 to the infundibulum and petuitary gland which here forms a 

 shorter hypophysis on the sella tursica than in reptiles. All 

 the lobes and cavities are immediately invested and lined by 

 the pia mater, and the dura mater lining the cranium forms 

 a distinct tentorium corebelli and a rudimentary falx ce- 

 rebri. 



The sensitive spinal nerves of birds have their ganglia 

 larger, and approximated more nearly to their origins than in 

 reptiles, and from the retraction and high termination of the 

 spinal chord, as well as the comparative magnitude of the 

 legs in this class, the posterior ganglionic enlargement is re- 

 markable for its size, and at this place the motor and sensitive 

 roots pass out through separate foramina of the numerous 

 anchylosed sacral vertebrae. The twelve pairs of cranial nerves 

 are here distinct as in reptiles and mammalia. The olfactory 

 nerves still provided with distinct glandular tubercles (98. A. 

 /,) arise from two transverse medullary bands (98. A. h,) ex- 

 tending, as in mammalia, to the fissure of Sylvius, and pass 

 forward through the narrow tapering osseous canals formed 

 by the frontal and ethmoid bones. The optic nerves (98. A. 

 i y ) corresponding in magnitude with the large eyes and the 

 optic lobes from which they originate, unite before the hypo- 

 physis, and partially decussate their numerous interwoven 

 component fasciculi before proceeding to the optic foramina. 

 The motores oculorum pass to the three innermost recti mus- 

 cles and the inferior obliquus of the eye, the trochlearis to 

 the superior obliquus, and the abducens to the rectus exterior 

 muscle, as in other classes. The large trigeminus sends its 

 ophthalmic branch to the upper parts of the face and nose, 

 the superior maxillary branch to the sides of the face and 

 upper mandible, and the inferior maxillary branch chiefly to 

 the lower jaw ; the two last branches exhibit a dental distri- 

 bution in the serrated mandibles of many aquatic birds, as in 

 the toothed jaws of quadrupeds. The smallness of the 

 facial nerve corresponds with the immobility and insensibility 



