138 Drs. F. W. Mott nail W. D. Halliburton.  [Nov. 14, 
The diagram represents the cerebrum as seen from above. The fissure of 
Sylvius is easily recognisable ; beneath this is a well-marked parallel fissure, — 
and above it is the sulcus known as the lateral fissure. In the frontal region 
the best marked fissure is the sulcus rectus. On the mesial surface of the 
hemisphere (not shown in the diagram) the intercalary fissure and the © 
calcarine fissure are the most prominent sulci. 
An interesting morphological point is, which of the smaller fissures in the © 
fronto-parietal region corresponds with the central fissure (fissure of Rolando) 
of higher animals. These are four in number, and are labelled 1, 2, 3, 4 in the 
diagram. It will be noticed that these differ in length, and to some extent in 
position in the two hemispheres; the drawing of the right hemisphere, how- 
ever, more accurately represents their average size than the left. Fissure 4 
is too far forward, and fissure 3 is too far back for the Rolandic fissure; and 
our experiments have given us no certain answer as to whether fissure 1 or 2 
is the fissure of Rolando, for the excitable area extends behind both. 
Sherrington and Griinbaum have shown that, in the higher apes, the 
excitable cortex does not extend behind the Rolandic fissure. In one experi- 
ment we have performed on one of the lower apes (Aacacus rhesus), we have 
found by the unipolar method the same to hold. So far as physiological 
enquiry can give any reply to morphological homologies, we feel inclined to 
regard fissure 2 rather than fissure 1 as the representative of the central 
sulcus, since behind it movements are evoked with greater difficulty than in 
front of it. 
The precise posterior boundary of the excitable cortex is very difficult to 
determine with accuracy, and is variable in different animals. Our diagram 
(Plate 2, fig. 1) represents the extreme posterior boundary, but in one or two 
instances it was somewhat more anterior. The cayse of the greater difficulty 
of evoking movements here becomes clear on histological examination ; the 
large motor cells become more scattered as one passes backwards. We 
obtained the most energetic movements from this posterior region by placing 
the electrodes 3 mm. apart, and applying them in a longitudinal direction. 
Very marked results (extension of arm and fingers) were in this way obtained 
from the oval area so marked in the figure. This may be explained on the 
hypothesis that the series of psychomotor cells presiding over this particular 
movement are arranged in a longitudinal direction, so that when the electrodes 
_ are applied in the manner stated the current spread over the whole of the 
excitable area of cortex representing the extension of arm and fingers. 
The diagram represents better than any amount of verbal description the 
disposition of the motor centres. The trunk and, more posteriorly, the leg 
areas are uppermost, and extend on to the mesial surface as far as the inter- 
