52 



BRAIN 



the corpora bigemina of Mammals, .and the optic thalami ; the 

 dorsal roof forms the epiphysis or pineal gland, the corpus callosum 

 and the anterior commissure, both of Avhich consist of bundles of 

 white nerve fibres and connect the right with the left hemisphere. 

 The ventral portion of the hemispheres consists of the corpora 

 striata, Avhich are masses of grey brain-substance, and of the olfactory 

 lobes, which mark the anterior end of the brain. 



The central canal, which runs through the spinal cord, is con- 

 tinued into the brain, and forms the fourth ventricle in the hind- 

 brain, extending dorsally into the cerebellum ; and is then continued 

 as " aquEeductus Sylvii " through the midbrain, with lateral exten- 

 sions into the optic lobes. The dilatation of this canal in the 

 thalamencephalon is the third ventricle : it extends ventrally 

 towards the hypophysis as the infundibulum, in a similar Avay 



Verticai, section in the 

 middle line through 



THE BRAIN OF A DuCK. 



Enlarged. (After H. 

 F. Osborne.) 

 J, Right olfactory nerve ; 

 JI, Right optic nerve and 

 chiasma ; acm, Anterior 

 commissure ; cal. Corpus 

 callosum ; cere6, Cerebel- 

 lum ; It, Lamina termin- 

 alis ; /?)i, Foramen Mon- 

 ro! ; Ixnn, Right hemi- 

 sphere; ?tjj/i, Hypophysis ; 

 inf, Infundibulum ; pew, 

 Posterior commissure ; 

 pn, Epiphysis or pineal 

 gland. 



hem^ 



en 



a em 



dorsally towards the epii:)hysis, and communicates through the 

 foramen of Monro with the second and first ventricles ; these being 

 the cavities of the two hemispheres,. 



The hypophysis cerebri or pituitary body is lodged in the 

 " sella turcica," a niche or recess formed by the anterior and 

 posterior basisphenoid bones. This peculiar body is probably the 

 degenerated remnant of a special sense-organ in the mouth of early 

 Vertebrata, it being developed partly as an outgrowth from the 

 roof of the mouth which fuses with a corresponding growth from 

 the brain and then loses its connexion with the mouth. 



The epiphysis cerebri or pineal body is the remnant of a 

 sense-organ, possibly visual, as it is still functional in many Lizards 

 possessing a lens, a retina-like accumulation of black pigment and 

 a nerve, but quite degenerated in all Birds and Mammals. 



The cereliellum of Birds is homologous only with the "Avorm" 

 or middle portion of the cerebellum of Mammals, the lateral lobes 

 being absent, althoiigh a pair of flocculi are present. Externally 

 it exhibits a number of transversa furrows, which divide it into 



