Her RICK, Gustatory Paths in Fishes. 413 



the floor of the thalamus (central tertiary tract). The dorsal 

 and mesial position of this derivative of the substantia reticular- 

 is is easily explained by the topographic features of the isth- 

 mus. Here the ventro-lateral region is occupied by the great 

 conduction paths between the oblongata and the mid-brain — 

 the tractus tecto-spinalis, tractus lobo-spinalis, lemniscus, etc. 

 The dorso-lateral region is occupied by the tuberculum acusti- 

 cum and its cerebellar connections. The enlarged secondary 

 gustatory terminal is prevented from growing ca\idad by the 

 great cerebellar crest and tuberculum acusticum. It must, 

 therefore, grow upward, inward and forward into the optocoele. 

 In this position it appears typically in all teleosts. But when 

 still more enlarged, as in cyprinoids, further growth in this 

 direction being prevented by the valvula cerebelli, which is also 

 very large in these fishes, it is forced to grow outward until it 

 appears as a superficial eminence cephalad of the tuberculum 

 acusticum and dorsally of the great ventro-lateral conduction 

 paths just referred to (Fig. 3). 



The secondary gustatory nucleus does not, however, com- 

 prise the whole of the substantia reticularis grisea of this region 

 of the isthmus. For caudo-mesially of this nucleus at the level 

 where the secondary gustatory tract enters it from the lateral 

 side of the oblongata is another considerable cellular area which 

 represents a less highly specialized portion of the same sensory 

 field. This also borders the ventricle, in some types forming a 

 considerable projection into it from the lateral wall immediately 

 caudad and ventrad of the commissure of the secondary gusta- 

 tory nuclei. Its anomalous position is brought about by the 

 same forces which were discussed above in connection with the 

 secondary gustatory nucleus. It is in very intimate relation 

 with the motor V nucleus which lies ventro-laterally of it and it 

 apparently is the chief medium of communication between the 

 various sensory pathways and that nucleus. Its neurones, in 

 other words, are like the others of the substantia reticularis 

 grisea in being of the tertiary sensory type and discharging into 

 a motor field, in this case the V nerve. It may therefore be 

 termed the substantia reticularis grisea ttigemini. 



