5IO 



COMPARATIVE ANATOMY 



the brain, the posterior commissure, marks the boundary between dien- 

 cephalon and mesencephalon. A dorsal commissure in the roof of the 

 midbrain connects the optic lobes. 



In addition to the commissures, the fibers of two cranial nerves, to 

 optic and the trochlearis, cross the median plane to form chiasmas. 

 The optic chiasma is ventral and just anterior to the infundibulum. The 

 trochlearis chiasma, which occurs in all vertebrates except cyclostomes, 

 lies in the dorsal constriction which separates mes- and metencephalon. 



CENTRAL SULCUS 



LENTIFORM NUC.-;<^ 



^i^^ ^Ovj THALAMUS 



^-OCCIPITAL POLE 



_i^ DENTATE NUC. 



NUC. VESTIBULARIS 

 y^NUC. GRACILIS AND CUNEATUS 



STRIATED 



MUSCLE ■ 



FIBER 



SOMATIC MOTOR NEURONE 



J r/*- 1 IN I « Pi 



!j CORPUSCLE 



SKIN 

 MUSCLE SPINDLE 

 ■ASSOCIATION NEURONES 



Fig. 427. — A diagram of the chief efferent tracts of the brain in relation to the spinal 

 cord. (Redrawn after Rasmussen s "Principal Nervous Pathways," The Macmillan 

 Co.) 



The dorsal commissure is lacking in fishes. In amphibia and reptiles 

 a dorsal pallial or hippocampal commissure, connecting right and left 

 hippocampi, adds a third to those located in the terminal lamina of the 

 brain. In monotremes and marsupials both anterior and posterior 

 pallial or hippocampal commissures occur. The corpus callosum of 

 placental mammals is a new commissure — possibly derived from the 

 anterior pallial — which connects the two halves of the neo-cortex. Its 

 enlargement is correlated with that of the cerebral cortex. 



Mesencephalon. The midbrain vesicle changes in phylogenesis 

 less than any other division. 



The mesencephalon of cyclostomes is peculiar in having a chorioid 

 plexus. In cyclostomes, as in fishes, the optic lobes of the midbrain are 

 the reflex centers of the optic nerve, such reflexes being mediated by 



