MORPHOLOGY OF THE FOREBRAIN 469 



amus. What the exteroceptive functions of the epithalamus were 

 in primitive vertebrates it is now difficult to determine, for there 

 have doubtless been important changes of function here correlated 

 with the degeneration of the epiphysis and parietal organ. But 

 we know that in some existing lower vertebrates there is a broad 

 fibrous connection between all parts of the epithalamus on the one 

 hand and both the optic centers and the underlying central grey 

 of the thalamus on the other hand, so that the whole epithalamus 

 is now very probably a correlation center for olfacto-optic and 

 olfacto-somaesthetic impulses, mainly no doubt related with the 

 reflexes of feeding and oral sensibility, as E dinger has suggested. 

 The efferent path from this exteroceptive olfactory center passes 

 to the ventral motor lamina by the shortest possible path through 

 the fasciculus retroflexus of Meynert. 



The remainder of the thalamic part of the dorsal lamina is 

 devoted to non-olfactory somatic sensory correlations, and imme- 

 diately contiguous with these centers motor correlation tissue 

 of the ventral lamina type is differentiated forward beyond the 

 primary limits of the ventral lamina, i.e., beyond the rostral end 

 of the sulcus limitans, thus ultimately obliterating this sulcus 

 in the rostral part of the diencephalon. The sulcus diencephalicus 

 medius marks the dorsal boundary of this forward extension of 

 the somatic motor correlation tissue and therefore is a secondary 

 extension of the sulcus limitans, though not a part of it. Thus 

 are formed the rostral end of the pars ventralis thalami and, 

 farther forward, the corpus striatum complex. 



That this division of the thalamus by the sulcus limitans and 

 sulcus medius into dorsal afferent and ventral efferent parts is 

 fundamental and characteristic of the vertebrate phylum as a 

 whole is shown by a survey of the comparative anatomy and em- 

 bryology of the thalamus, though it is often secondarily obscured 

 in the adult. It is very evident in the generalized fishes, and, as 

 I have shown in this paper, in both embryonic and adult amphib- 

 ians and reptiles. Ramon y Cajal has clearly demonstrated 

 the same relation in the rodents ('04, p. 774). It is equally 

 clear in early human embryos, as will appear beyond (p. 475). 

 It has, in fact, long been recognized that the rostral end of the 



