Turner, Morfhologv of the Avian Brain. 53 



the fourth ventricle, which is entirely covered by the epen- 

 cephalon. 



EXPLANATION OF TABLE V (SEE P. 86). 



" Length" is a contraction for "Ratio of the length of the inetencc- 

 plialoii to the length of the brain." 



" Breath " is a contraction for " Ratio of the greatest breadth of the 

 inetencephalon to the length of the brain." 



" Depth'" is a contraction for " Ratio of the greatest depth of the 

 metencefhalon to the length of the brain." 



All ratios are expressed in hundredths of the length of the brain. 



The length of the brain is the distance between the cephalic extrem- 

 ity of the prosencephalon and the center of the rhombic depression 

 which lies immediately caudad to the twelfth nerve root. 



Pineal body. — This is always present (Plate V, Figs. 5, 

 6, 8, 10). It is a small sub-conical body, which lies in the 

 caudal V of the prosencephalon. The apex of this cone 

 points ventrad. From it the habenula passes ventrad, around 

 the cephalic portion of the epencephalon, to the habena. 

 The pineal gland is intimately connected with the dura and 

 is liable to be reinoved with it. • 



Infundihulum (Plate V, Figs. 9, 13). — On the ventral 

 surface of the brain, between the diencephalon and the 

 metencephalon, lies the tuber cinerium, which is pierced by 

 the infundihulum. 



Pituitary body. — Immediately ventrad to the infundihu- 

 lum, lies the pituitary body. It is encased in a special cavity 

 of the skull. 



NERVES. 



The external course of the nerves has been so well de- 

 scribed by Prof. Elliott Coues(') that it is not necessary for 



1 The cranial nerves are twelve pairs, as in mammals, the highest vertebrate number. 



1. The olfactory, nerve of special sense (smell); origin from rhinencephalon ; exit 

 from cranial cavity by olfactory foramen, high up in the orbital cavity; conducted along 

 a groove to find escape between the perpendicular and lateral plates of the ethmoid i 'to 

 the nasal chambers; distributed to the investing mucous membrane of the septal and 

 turbinal bones of the nose. The exit is thiough a sieve-like or cribi/orm plate only in 

 Apteryx and Dinornis (Owen . 



2. The optic, nerve of special sense (sight); origin from optic lobe and thalamus(?); 

 of great size and forming a chiasm (decussation^ with its fellow; exit by optic foramen, a 

 large hole in the back of the orbital cavity between the centers of orbito-sphenoid and 



