4 JOURNAL OF COMPARATIVE NEUROLOGY. 
occipital lobe. It separates the temporal lobe from the occi- 
pital. At its origin it consists of two parts, an upper limb 
which arises from the superior margin of the occipital lobe and 
a shorter, lower limb which commences a little below the centre 
of the occipital lobe. These two limbs unite midway between 
the upper and the lower margins of the cerebrum. The calca- 
rine fissure then runs at first downwards and forwards to the 
base of the hemisphere. It then passes forwards and slightly 
upwards, and terminates in the dentate fissure. 
The calloso-marginal sulcus extends from the middle of the 
great longitudinal fissure almost to the apex of the frontal lobe. 
At first it curves downwards, then courses directly forwards, 
and finally curves upwards to terminate in the great longitudinal 
fissure. Before curving upwards it gives off a short inferior 
limb. 
The Frontal Lobe. 
The frontal lobe presents three surfaces, external, internal 
and orbital. The external surface is bounded inferiorly by the 
Sylvian fissure and posteriorly by Rolando. The external 
surface presents two fissures, the precentral and the horizontal. 
The precentral fissure is vertical in direction, and lies midway 
between the apex of the lobe and the fissure of Rolando. 
Superiorly it terminates by dividing into an anterior and a 
posterior limb. 
The horizontal fissure lies a little posterior to the apex of 
the frontal lobe. It runs at right angles to the precentral sul- 
cus. Posteriorly it curves slightly upwards. 
The precentral and the horizontal fissures divide the 
frontal lobe into three convolutions. Of these the most pos- 
terior one corresponds exactly to the ascending frontal gyrus of 
man being bounded anteriorly by the precentral sulcus, inferi- 
orly by Sylvius and posteriorly by Rolando. Just what the 
remaining two gyri are is difficult to say. BiscHorF declares 
that in the gorilla and the orang Broca’s convolution is but ill- 
developed ; and according to Panscu the third frontal convolu- 
tion is absent in all other apes. If Panscu be correct in his 
