DISSECTION OF THE BRAIN, OR ENCEPHALON. 247 



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

 the hemisphere, and abuts on tlie 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 hemisiihere, and reaches its upper aspect (Plate 34). Here it is reflected back- 

 wards 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 convolu- 

 tion 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. Be- 

 ginning at the crucial fissure (being sometimes connected to the postero-marginal convolu- 

 tion 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, T. 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, \. O) lies external to the preceding. Be- 

 ginning 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 fissiure, 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-iNPEEiOR LoBE. The sulci of this lobe are numerous and small, and it 

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

 ever, 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. 

 Sejiarate 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 fornicatus. 



The Gyrus Fornicatus (Fig. 31). — This is comi>arable to a lobe,* rather than to a 

 convolution. It is disposed in a great curve, or arch, from which it is named. It begins 

 at the forepart of the under siu'face of the hemisphere, in front of the lamina cinerea, 

 and here it is narrow and pointed. It bends round the anterior extremity (genu) of the 

 corpus callosum, acquiring at its point of reflection a great increase in thickness. It 

 passes backwards above the corpus callosum, and below the antero-marginal and postero- 

 marginal convolutions. From the former body it is separated by the fissure of the corpus 

 callosum, while the calloso-marginal fissure (great limbic fissure of Broca) separates it from 

 the marginal convolutions above. In this part of its coiirse the gyrus is distinctly divided 

 into two tiers by a fissure that traverses it in its length. Posteriorly this fissure becomes 

 very shallow, and the gyrus, losing its double character, turns roimd the posterior end 

 (splenimn) of the corpus callosum and reaches the tentorial surface of the hemisphere. 

 At this point it becomes slightly constricted ; and after being connected with the convolu- 



* Paul Broca (Anatomic comparce ihs circonrohUions cerebndes) considers that this part of the 

 hemisphere represents not merely a lobe, but several lobes — that it is, in fact, the eoLuivalent of all 

 the rest of the cerebral cortex. He accordingly divides the surface of the hemisphere primarily into 

 two great divisions — the great liinbic lobe (gyrus fornicatus) and the conrolutionary mass. 



