42 I()ii{\Ai, oi- Com I'AKA ri\ K Neukoloov. 



velops rapidly. As this body grows cephalad and dorsad it 

 will separate the optic lobes. Now let the cerebral hemi- 

 spheres increase in length. Let the lateral and dorsal por- 

 tions of each hemisphere grow caudad much more rapidly 

 than the mesal and the ventral. This will cause the caudad 

 portion of each hemisphere to levolve toward the meson, 

 and, at the same time, to over-ride the optic lobe. A com- 

 parison of the different lobes found in the avian brain with 

 the corresponding lobes of the reptilian encephalon shows 

 that this mesal revolution has actually been performed. 



RHINENCEPHALON. 



Size. — Compared with that of other Sauropsida, the 

 avian rhinencephalon is quite small. In most birds, its 

 length is equal to about lo per cent, of the length of the 

 brain; but in some types {Ardeidcv, Anatidcv) , this ratio 

 rises to i8 or 20 per cent.; while in others (Corvidiv) , \t 

 falls to about 6 per cent.(') (see Table I). 



.Form. — With regard to position, the avian rhinencephala 

 fall into two classes. Into the first class fall those which 

 project beyond the cephalic end of the prosencephalon; into 

 the second class fall those which do not thus project.( ') 



Each rhinencephalon of the first class consists of two 

 elongated sub-ellipsoidal bodies which arise side by side, 

 either at or immediately ventrad to the cephalic end of the 

 prosencephalon, and project cephalad. Each of these lobes 

 contains a ventricle, which, I think, is always continuous 

 with the lateral ventricle of the cerebral hemisphere. 



Each rhinencephalon of the second class consists of one or 

 two short sub-ellipsoidal bodies which are partly imbedded 



1 After carefully studying the cerebrum of European birds, A Bumm remarks: " In 

 swimming birds, the olfactory lobes are well developed ; in wading birds (Sumpfvogel), 

 they are moderately developed ; in all other cases, they are only slightly developed." He 

 then adds the following table : 



Ratio of the iveight of the Rhinencephalon to the weight of the Prosencephalon. — In 

 the goose as i : 67.0 ; in the snipe as i : 84.5 ; in the buzzard as i : 543.0. Op. cit., p. 436. 



2 In European birds, these two types of rhinencephalic structure have been recog- 

 nized by A. Bunim. Op. cit., p. 435. 



