526 CEEEBRUM. 



across the lateral convex surface of the hemisphere. It commences 

 above, behind the vertex, near the great longitudinal fissure, and 

 passes downwards and forwards to end close behind, but above, the 

 bifurcation of the fissure of Sylvius, into the posterior limb of which 

 it has been seen, though very rarely, to open. Its position and direction 

 are such that the fissures of the two sides, seen from above, form a 

 V-shaped line, open in front. It is rarely interrupted in its course, and 

 is very uniform in man and most of the primates. It appears early in 

 development, about the fifth month. 



This fissure separates the frontiil from the parietal lobe, and its 

 position on the surface of the hemisphere varies with the degree of 

 development of the frontal lobe. The parallel convolutions which it 

 separates are named respectively the ascending or transverse frontal 

 and ascending parietal convolutions. 



The Parieto-occipital fissure, or perpendicular fissure (fig. 370, par. 

 oc.f.), is best marked on the median surface of the hemisphere, where it 

 appears as a deep cleft extending downwards and a little forwards from 

 the margin of this surface to near the posterior extremity of the corpus 

 callosum. On the convex surface it is continued transversely outwards 

 for a variable distance, generally about an inch, as the external ^^arieto- 

 occipital fissure. This fissure is taken as the division between the 

 parietal and occipital lobe. The size of its external portion depends 

 (inversely) on the size of the convolution which curves round its outer 

 extremity and connects the parietal with the occipital lobe. In conse- 

 quence of the development in man of this and similar connecting 

 convolutions, this fissure is much less marked in the human brain than 

 in that of the higher apes. It appears about the fourth or fifth month. 



Outer Surface of the Hemispheres. — Lobes. — These are deter- 

 mined chiefly by fissures on the outer, which are for the most part 

 absent on the median surface. It is convenient therefore to consider 

 them in relation to the outer surface only, and subsequently to describe 

 the median surface as a whole. 



The Frontal Lobe (fig. 370) is the anterior portion of the brain 

 in front of the fissure of Sylvius at the base, and of the fissure of 

 Rolando on the outer side. On the median surface there is no corre- 

 snonding demarcation. The inferior surface of the frontal lobe, which 

 is in contact with the orbital plate, is called the orbital surface ; the 

 upper arched aspect is the frontal surface. 



FitoxTAL SURFACE. — Tlic convoluHous are four in number, three antero- 

 posterior, one above the other, and one transverse behind them. 



The ascencling or transverse frontal convolution, (asc. fr. c.) is placed in 

 front of the fissure of Eolando, which it thus bounds. Below, it com- 

 mences just above and behind the bifurcation of the fissure of Sylvius, 

 and thence courses upwards and backwards to the margin of the great 

 longitudinal fissure. Commonly above, and almost invariably below, 

 it is connected with the convolution (ascending parietal) behind the 

 fissure of Rolando, and thus that fissure is isolated. 



The portion of the frontal surface anterior to this convolution is 

 occupied by complex convolutions running more or less in an antero- 

 posterior direction, and usually to be distinguished into three, an upper, 

 middle, and lower. These may or may not arise superficially from the 

 ascending frontal convolution ; they are usually in their course con- 

 nected one with another by secondary convolutions. 



