Vili JOURNAL OF COMPARATIVE NEUROLOGY. 
cornu of the lateral ventricle is not necessary for the existence of a 
calcar avis. Thus we find a free calcar in many hemispheres (those of 
Orycteropus, Thylacinus, Pteropus, for example) in which there is no 
posterior cornu. But in most mammals the calcar becomes hidden by 
a great mass of fibres (compare most Carnivores and Ungulates), and 
cannot therefore be said to exist as a projection im the ventricle; and 
yet in many large Carnivores (Phoca for instance) and Ungulates 
(Camelus) a small posterior cornu of the lateral ventricle makes its 
appearance, and with it a typical calear again becomes exposed as in 
the Primates. 
It would be strange indeed if the most constant and stable sulcus 
of the mesial surface of the hemisphere of most mammals should en- 
tirely disappear in the Primates, to be replaced by another sulcus pre- 
senting identical relations to the lateral ventricle and a similar develop- 
mental history, but without being homologous. There is an over- 
whelming mass of evidence to show that the vertical part of the sulcus 
generally called ‘‘splenial” is the direct homologue of the calcarine 
sulcus of the Primates. 
In many mammals (such as the Lion) the tension of the growing 
infracalcarine neopallium is relieved chiefly by the downward extension 
of the calcarine sulcus toward the posterior rhinal fissure, but also 
partly by certain irregular and inconstant compensatory sulci behind 
and parallel to this extension. In the Primates, however, the calcarine 
sulcus becomes very obliquely placed, not only because the occipital 
region of the hemisphere becomes caudally extended above the cere- 
bellum, but also because the elongating corpus callosum pushes back, 
as it were, the pericalcarine neopallium ; and as a result of this obliquity 
the sulcus cannot be prolonged towards the rhinal fissure, as happens 
in the Carnivora and Ungulata, so that the compensatory sulcus, which 
is known as the ‘‘collateral’”’ sulcus, attains a greatly enhanced import- 
ance, and fulfils the rdle of the ventral extension of the calcarine 
sulcus. 
A study of the variable collateral sulcus in the brain of Man and 
the Apes clearly shows its compensatory-calcarine nature. 
Another result of the occipital prolongation of the hemisphere is 
that the calcarine sulcus becomes widely separated from the intercalary 
(calloso- marginal) sulcus, to which it is joined in most mammals. The 
stages in this separation are well shown by comparing, say, the brain 
of a typical Carnivore with those of the Daubentonia and the Lemurs. 
As the result of this separation a new set of mechanical conditions pre- 
vail in the area between the calcarine and the calloso-marginal sulci; 
and, to further complicate matters, the acuate sulcus formed on the 
dorso-lateral aspect of the hemisphere by the lateral and post-lateral 
sulci (é. e. the intraparietal and its ramus occipitalis transversus re- 
spectively) becomes more and more acutely flexed as the occipital pro- 
longation occurs, so that in the Cebide the sharp-pointed apex of the 
V-shaped sulcus so-formed extends toward this region of the mesial 
wall, which is, for the reasons just mentioned, already in state of ‘‘un- 
stable equilibrium,” so to speak. As the result two sulci (which may, 
