36 GENERAL HISTORY OF BIRDS. 



may be disclosed by slightly parting the hemispheres behind. Transversely, 

 the cerebellum is ridged and presents for examination a median and two lateral 

 regions. The flexure of the brain mass on the axis of the cord is well-marked 

 in birds, and is present to an extent to hide the medulla by the curvatui'e of 

 the cerebellum, from which latter it descends more or less vertically. The 

 pons varolii is absent, as is also the corpus callosum. In the brain-case we 

 find the basisphenoid bones forming a "sella turcica," and in this recess is™ 

 lodged the hypophysis cerebri or pituitary body, while the pineal gland, or! 

 epiphysis cerebri, has already been mentioned above. This latter body isj 

 doubtless the remnant of a third visual organ, still functional in not a few ex- 

 isting lacertilians. 



Four ventricles exist in the bird's brain, the first and second being the 

 cavities of the two hemispheres; the third is the dilatation of the central canal 

 of the spinal cord after it arrives in the thalamencephalon, which canal con- 

 stitutes the fourth ventricle in the hind-brain, that extends dorsally into the 

 cerebellum, and, as the aqueduct of Sylvius, passes through the midbrain into 

 the optic lobes upon either side. The communication with the first and second 

 ventricles is through the foramen of Munro. 



There are, as usual, twelve pairs of cranial nerves, and the spinal cord, the 

 latter being the continuation of the medulla oblongata, extends through the verte- 

 bral canal well into the caudal vertebrte in most birds. From it are thrown ofl' the 

 spinal nerves, which arise from the medulla of the cord by ventral and dorsal 

 roots, the former being made up of motory fibres, and the latter of sensory 

 ones. In birds, we find a brachial as well a sacral plexus, the former being 

 composed nf some four or five of the inferior cervical nerves, and the latter of 

 a number of pelvic spinal nerves. 



A sympathetic nervous system also exists in Aves. It supplies mainly the 

 vascular, alimentary and reproductive systems, and consists of the visceral 

 branches of the cerebro- spinal nerves. 



The sense of touch is highly developed in various parts of the body in 

 most birds, while divers structures in some species are entirely devoid of it, as 

 the mandibles and feet. Snipe, however, appear to be sensitive at the ex- 

 tremity of the upper bill, and birds with fleshy feet, no doubt, can appreciate, 

 contact with objects brought lightly in touch with them. Most birds are more 

 or less lacking ia a keen sense of smell, while the senses of hearing and sight 

 are both remarkably well developed. The external ear is extremely rudi- 

 mentary, while the passage of the external auditory meatus is well marked 

 and surrounded by a circlet of feathers. A tj^mpanum is found within, which 

 is connected with the fenestra ovalis by means of the osseous columella. In 

 the internal ear, the cochlea is very large in birds, and the Eustachian tubes 

 open anteriorly in front of the basisphenoid bone. 



