ANATOMY OF A CYCLOSTOME BRAIN 659 



trails thalami. The horizontal limb of this sulcus is our sulcus 

 medius. Johnston states ('12, p. 349) that this is a part of his 

 sulcus limitans hippocampi and that Herrick's sulcus ventralis 

 is the ventral part of his sulcus hypothalamicus. Neither of these 

 statements is supported by our present examination. We have 

 already seen that the sulcus medius and the limiting sulcus of 

 the hippocampus (sulcus subhippocampalis) are separated by the 

 subhippocampal lobe; and our sulcus ventralis is a longitudinal 

 groove which cuts across the hypothalamic sulci at almost a right 

 angle. 



The opinion was expressed by the senior author (Herrick '10, 

 p. 471) that the sulcus medius and the sulcus ventralis are com- 

 parable with the sulci so named by him in the Amphibia. The 

 solution of this question must be reserved until fuller knowledge 

 of the functional connections of the related parts of the cyclo- 

 stome brain is obtained, and the terms are here used in a descrip- 

 tive sense merely. 



In higher brains the thalamus proper is clearly divided into 

 a dorsal part which receives the lemniscus, optic tracts, etc., and 

 is in the main a cortical dependency, and a ventral part more 

 directly concerned with motor or emissive functions. As far down 

 the animal scale as the Amphibia this distinction is very evident, 

 though in the latter case the sensory correlation centers of the 

 dorsal part of the thalamus have small cortical connections. 

 How far this type of differentiation prevails in the thalamus of 

 fishes must await further study. To us it appears not improbable 

 that even in cyclostomes the medial and ventral lobes of the thala- 

 mus as here described have been differentiated in accordance 

 with the same functional factors as are evident in the dorsal and 

 ventral parts respectively of the amphibian thalamus. 



The status of the subhabenular lobe is obscure. The meager 

 data at hand suggest that it is a habenular dependency, but 

 whether it belongs primarily in the thalamus or the epithalamus 

 must be determined by further study. Tretjakoff ('09, p. 732) 

 describes fibers of the stria meduUaris system from the secondary 

 olfactory area as ending in the thalamus (probably in our sub- 

 habenular lobe), partly uncrossed and partly after decussation 



