RHINENCEPHALON OF DELPHINUS 501 
According to Edinger and Wallenberg, there is a tract, the 
basal olfactory bundle, running from the olfactory bulbs and 
peduncles in a direct path to the region of the corpora mamil- 
laria, and part of the fibers may go even to the ganglion inter- 
pedunculare. 
These parts of the mammalian brain, which are concerned in 
the conduction of olfactory and associated impulses vary greatly 
in their form and position, and in their size relative to surround- 
ing structures, in the different species. This variation depends 
upon two main circumstances. The first is the degree of develop- 
ment of the olfactory mechanism itself. The second is the posi- 
tion that the animal has attained in the scale of intelligence, this 
being connected, usually, with the size of the neopallium. In 
the higher forms, there is increasing development of the neo- 
‘pallium, and consequent overshadowing of the primitive parts. 
As the lower orders of mammals possess, in general, a well- 
developed olfactory sense, without any great development of the 
neopallium, they present a more easily understood olfactory 
mechanism. The study of such forms in recent years has made 
clear the fundamental olfactory portions of the brain. With 
the increasing development of the neopallium and associated 
structures (frontal and temporal lobes, corpus callosum) and 
often the decreasing importance of the olfactory structures, a 
much more intricate arrangement has been arrived at, as in the 
human brain. An intermediate condition, where there is a 
medium development, both of the olfactory apparatus and of 
the neopallium, is found in many animals, such as terrestrial 
Carnivora and the ungulates. 
These three grades are illustrated in figures 1, 2, and 3, in the 
drawings of the base of the brain of the rabbit, calf and man. 
These demonstrate the external differences between macros- 
matic and microsmatic forms, and the variation which may 
exist in the relative extent of the rhinencephalon within the 
macrosmatics. 
Partly perhaps as the result of this morphological variability, 
one often finds a diversity of meaning attached to one term, 
as used by the various investigators working on comparative 
