350 PHYSIOLOGICAL SERIES. 



The brain of the Odontoceti is peculiar in that the 

 olfactory apparatus, which develops in the early embryo 

 just as it does in other mammals, becomes entirely aborted 

 before the time of birth. As a result the olfactory bulb 

 and its peduncle completely disappear; the cortex of the 

 tuberculum olfactorium atrophies so that a broad expanse 

 of corpus striatum comes to the surface immediately in front 

 of the optic chiasma on the basal surface of each hemisphere: 

 the pyriform lobe becomes greatly reduced in size and altered 

 in structure ; the hippocampus, including the fascia dent ata, 

 and ibrnix become reduced to very diminutive proportions; 

 and the anterior commissure becomes greatly attenuated, 

 or may even disappear entirely. This abortion of the 

 apparatus of smell is undoubtedly to be attributed to the 

 purely aquatic habits of the Cetacea ; and it is of interest 

 because in other purely aquatic animals, such as Fish, 

 Amphibia, and Reptiles, such a complete, disappearance of 

 the mechanism of smell cannot occur because there is no 

 neopallium, which as a il sensorium commune" can make 

 good the absence of the sense of smell. 



The peculiar spherical shape of the cerebral hemisphere 

 is to be attributed chiefly to this waning importance of the 

 rhinencephalon. 



The primitive cerebrum of all mammals consists essen- 

 tially of two parts : a basal olfactory part and a dorsal 

 neopallium. The relatively greater expansion of the latter, 

 which the dwindling of the former must entail, naturally 

 produces the marked ventral bending of the anterior and 

 "temporal" poles of the hemisphere, so that when the 

 enormous growth of the neopallium, which distinguishes 

 this Order, occurs the hemispheres become moulded by this 

 early bias and assume a shape which appears roughly 

 spherical from above and kidney-shaped in profile. The 

 " hilus'' of the kidney-shape forms the " Sylvian fissure" 

 (using this term with the reservation already adopted tor 

 the Garni vora and Ungulata). 



The hemispheres in all the Cetacea become complicated 

 in a most intricate and highly elaborate manner by the 

 development of innumerable sulci. But the essential plan 

 of the chief sulci of the hemisphere is a modification of the 



