DISSECTION OF THE BRAIN, OR ENCEPHALON. 253 



inferior lobe ; and the fourth is directed forwards into the anterior lobe. In the first 

 part of its course the fourth branch separates the adjacent convolutions of the anterior 

 and postero-inf erior lobes, being itself separated by a bridging convolution between these 

 lobes from the lower extremity of the lateral fissure. 



To the outer side of the outer olfactory tract, at the point from which these branches 

 of the Sylvian fissure radiate, there is a minute nodular convolution that is partially 

 or entirely concealed from view until the adjacent convolutions are slightly separated. 

 This seems to foreshadow the convolutions of the insula of man. 



The Anterior Lobe presents four convolutions : — 



1. The First Anterior Convolution (Plate 33, 1. A) is seen on the under surface of the 

 lobe. It lodges the olfactory peduncle in the olfactory fissure ; and when the peduncle 

 is in position it shows an inner and an outer part, the former occupying the position of 

 the gyrus rectus of human anatomy. 



2. The Second Anterior Convolution begins on the under surface of the lobe (Plate 33, 



2, A), external to the preceding. It runs forwards and upwards round the extremity of 

 the hemisphere, and abuts on the antero-marginal convolution (Plate 34). 



3. The Third Anterior Convolution begins at the under surface of the lobe (Plate 33, 



3. A) to the outer side of and behind the preceding. It curves upwards and forwards 

 across the hemisphere, and reaches its upper aspect (Plate 34). Here* it is reflected 

 backwards and inwards ; and, turning upon itself, it descends to near the point from 

 which it started, being connected at its termination by a bridging convolution to the 

 first convolution of the postero-inferior lobe. 



4. The Antero-marginal Convolution (Plate 34, A. M.) lies at the forepart of the 

 great longitudinal fissure, and is visible on both the upper and inner aspects of the 

 lobe. Beginning at the crucial fissure (being sometimes connected to the postero- 

 marginal convolution of the postero-superior lobe), it passes forwards at the edge of 

 the hemisphere, at the anterior end of which the first and second anterior convolutions 

 abut upon it. 



The Postero-superior Lobe comprises three convolutions : — 



1. The Postero-marginal Convolution (Plate 34, P. M.) extends along the margin 

 of the lobe, appearing on both its upper and inner surfaces, and lying in series 

 with the antero-marginal convolution of the anterior lobe. Beginning at the crucial 

 fissure, it passes backwards at the edge of the hemisphere, and reaches its tentorial 

 aspect. 



2. The First Oblique Convolution (Plate 34, I. O) lies external to the preceding. 

 Beginning in front, near the margin of the hemisphere, it passes obliquely backwards 

 and outwards, and curves round the extremity of the lobe to reach its tentorial surface. 



3. The Second Oblique Convolution (Plate 34, 2. O) passes with an oblique direction 

 between the preceding convolution and the great oblique fissure, and reaches the tentorial 

 Surface of the lobe (Plate 33), from which it seems to be in part continued by the outer 

 olfactory tract. 



The Postero-inferior Lobe. The sulci of this lobe are numerous and small, and 

 it is difficult to divide it naturally into convolutions. For convenience of description, 

 however, two convolutions may be recognised in it : — 



1. The First Postero-inferior Convolution (Plates 33 and 34, 1. P. I.) is four-sided 

 and contains within itself several short sulci. It lies behind the third convolution of 

 the anterior lobe, to which it is connected by a bridging convolution. 



2. The Second Postero-inferior Convolution (Plates 33 and 34, 2. P. I.) lies at the 

 posterior part of the lobe, above the outer olfactory tract ; and its posterior extremity 

 appears on the tentorial surface of the hemisphere. Like the preceding, it possesses 

 numerous minor sulci within itself. 



Directions. — There still remains for examination the inner surface of each hemisphere. 

 Separate the hemispheres as widely as possible along the great longitudinal fissure. At 

 the upper edge of this fissure there will now be seen the inner aspect of the antero- 

 marginal and postero-marginal convolutions already described, and between the lower 

 edge of these and the corpus callosum there lies a thick convolution — the gyrus 

 fomicatus. 



