FEATURES OF THE MAMMALIAN BRAIN. 439 



THE ENCEPHALIC SEGMENTS IN THE CAT. 



1130. The recognition of the several segments is less easy than 

 with the frog in some respects on account of certain features of the 

 mammalian encephalon. 



1131. Differences between the Brains of the Cat and the 

 Frog. In a general way these have been indicated already ( 1055, 

 1056) ; they may be more definitely stated as follows : 



(1) In the cat the ccelise are irregular in form and relatively 

 reduced in extent (Fig. 113 ; PL II, Fig. 4). 



(2) The parietes are relatively thicker, and more differentiated 

 as to both contour and structure (Fig. 113 ; PL II, Fig. 4 ; PL III, 

 Fig. 13). 



(3) With several segments the dorsal portion (roof) is much 

 more extensive than the ventral (floor), (Fig. 117 ; PL II, Fig. 4). 



(4) The longitudinal axis of the entire brain presents a decided 

 flexure, the convexity of which is dorsal and coincides in position 

 nearly with the mesencephalon (Fig. 88). 



(5) The dorsal portions of two segments (epencephalon and pros- 

 encephalon) are so greatly enlarged as to cover all the others except- 

 ing parts of the metencephalon and rhinencephalon, the extremes 

 of the series (Fig. 88, 104, 117; PL I, Fig. 1, 2). 



(6) In addition to the Amphibian commissures (ckiasma, postcommissura, prcecommis- 

 sura\ there are in the cat's brain more or less distinct fibrous fasciculi constituting other 

 commissures longitudinal (Crura cerebri, PI. II, Fig. 3, PI. Ill, Fig. 11 ; Crura olfactoria, 

 PL II, Fig. 3, 4 ; postpedunculi, praepedunculi) ; transverse (pons, Fig. 117 ; PI. II, Fig. 3, 

 pn. ; decussatio pyramidum, medipedunculi, Commissura fornicis, PI. IV, Fig. 14, cs.f.); 

 and oblique (fornix, Pi. IV, Fig. 14,/.). 



(7) Between the apposed surfaces of the thalami is established a connection, the medi- 

 commissura (Fig. 122; PI. II. Fig. 4, mcs.\ which occupies a large portion of the diacoelia. 



(8) The apposed surfaces of the hemispheres are connected along a slightly curved line 

 by a thick band of fibers, the callosum (Fig. 104, 115 ; PI. Ill, Fig. 13, PI. IV, Fig. 15, cl.), , 

 the largest and presumably the most important of the commissures. 



(9) The dorsal aspect of the mesencephalon presents a transverse furrow, distinguish- 

 ing a cephalic pair of lobes, the optici proper, from a caudal pair, the postoptici (Fig. 

 114; PI. Ill, Fig. 7); hence the bigeminum of the frog becomes the quadrigeminum of 

 the cat. 



(10) The lateral aspect of the diencephalon presents at least two elevations, postgenio 

 ulatum and praegeniculatum (PI. Ill, Fig. 7, 9, p,gn., prgn.). 



(11) Of the diacoelian roof, the membranous part (diatela, PL Til, Fig. 10) is relatively 

 more extensive, while the nervous part (postcommissura, PL II, Fig. 4, pcs.) is corre- 

 spondingly diminished ; concomitantly the conarium is attached near the caudal end of 

 the diencephalon and is retroverted so as to rest upon the mesencephalon. 



