280 C. JUDSON HERRICK 



tions of each. Some additional histological details will be added, 

 though no attempt has been made to give a complete account 

 of all of the types of neurons represented. The epithalamus 

 and hypothalamus are not included in this analysis, and the 

 telencephalic connections will be mentioned in only a few cases. 



The classification of primary cerebral centers and tracts 

 into afferent and efferent types is easily effected; but this be- 

 comes increasingly difficult (and unimportant) when applied 

 to correlation centers and tracts of higher orders. Neverthe- 

 less most correlation centers of the dorsal parts of the midbrain 

 and thalamus are evidently more intimately connected with the 

 lower afferent systems than with the lower efferent systems. 

 Conversely, the ventral parts of the midbrain and thalamus 

 are in more direct relation with the lower motor centers. 



In the midbrain the roof (tectum) is the receptive center 

 for sensory impulses coming in by way of the mesencephalic 

 V root and from the retina by way of the optic tracts, and for 

 afferent fibers of other systems by way of the spino-tectal 

 and bulbo-tectal lemniscus systems. The floor of the midbrain 

 (cerebral peduncle) and isthmus region give rise to efferent 

 fibers of the III and IV nerves and to the great descending 

 motor systems, and receive important tracts from the cerebel- 

 lum, from the tectum, from all parts of the diencephalon, and 

 from the cerebral hemispheres. The significance of its connec- 

 tions with the nervus terminalis and nervus parietalis is still 

 obscure. 



The epithalamus and hypothalamus are correlation centers 

 dominated by descending olfactory tracts. The pars dorsalis 

 thalami is a receptive center for various somatic sensory tracts 

 and it" discharges downward into the pars ventralis thalami, 

 cerebral peduncle, and hypothalamus; it also sends many fibers 

 (thalamo-frontal tracts) forward into the lateral wall of the 

 cerebral hemisphere. The pars ventralis thalami receives fibers 

 chiefly from the tectum mesencephali, from the pars dorsalis 

 thalami and from the lateral wall- of the cerebral hemisphere 

 by way of the lateral forebrain bundle. At its anterior end it 

 also receives fibers from the primordium hippocampi through 

 the tractus cortico-thalamicus and eminentia thalami (p. 290). 



