Turner, Morphology of the Avian lirain. 41 



exte;r\al form. 



Size. — Compared with the brains of other .Sauropsida, 

 the bird brain is quite large. It fills the entire cavity of the 

 skull. This cavity is relatively much larger in birds than it 

 is in other members of the same group. 



Compactness. — The most remarkable characteristic of the 

 avian brain is its compactness. The large prosencephalon 

 entirely covers the diencephalon, and may or may not cover 

 the rhinencephalon and mesencephalon. Along about three- 

 fourths of their mesal border, the two lobes of the prosen- 

 cephalon are compressed against each other. Near their 

 narrow cephalic end, the two hemispheres are slightly 

 divaricated; near their broad caudal end, they are strongly 

 divaricated(') (Plate V, Figs. 5, 6, 8, 10.) 



Into the caudal V^ thus formed, the cephalic portion of 

 the well-developed epencephalon is wedged so firmly that a 

 portion of it is crowded beneath the prosencephalon. The 

 metencephalon lies beneath the epencephalon, and is almost 

 completely covered by it (Plate V, Figs, i, 4; Mt.). 



Evohttion. — The compact eiicephaloii , the well-developed 

 epencephalon, the ventral tncsenccphalon, — all these character- 

 istics completely separate the avian from the reptilian type 

 of brain; yet it is easy to see how the first might have been 

 derived from the second. Allow me to refer to the form of 

 an alligator's brain.(') Viewing the dorsal surface of the 

 brain, we observe the following points. Between the small 

 prosencephalon and the poorly-developed epencephalon, lie 

 the two tangent sub-ellipsoidal lobes of the mesencephalon 

 In a brain of this type, suppose that the epencephalon de- 



1 A. BiMM transcribes Tiedemann as follows: " The avian brain resembles an ace of 

 hearts, whose apex is directed cephalad and whose base is directed caudad " ("Das 

 Grosshirn der Vogel," von A. Bumm, Zeitschrift (Ur Wissenschaftliche Zoologie, vol. 

 38. P- 431. 



Stieda makes a similar figure, and then adds : "In die vertiefte Basis des kartenherz- 

 formigen Grosshirn schiebt sich das Cerebellum." 



2 See "Notes on the Brain of the Alligator," by Prof. C. L. Herrick, jour of the 

 Cincinnati Nat. Hist. Soc , Vol, 12, PI. Vll, Figs. 1, 2, 3. 



