INNER CEREBRAL CONVOLUTIONS. 197 



Above, it blends commonly with the angular convolution, and is connected 

 to the middle occipital convolution by an annectant gyrus (e). 



The inferior ( 9 ), less well marked than the other two, forms part also 

 of the inner surface of the temporo-sphenoidal lobe. By the upper end 

 it is united to the third occipital convolution by an annectant gyrus (d). 



The parallel sulcus (P, fig. GO), named from its position to the Sylvian 

 fissure, extends from the lower end of the temporo-sphenoidal lobe to the 

 angular convolution. 



e. The convolutions of the central lobe ( (7, fig. 61), about six in number, 

 are straight for the most part, and are separated by shallow sulci : they 

 are directed upwards from apex to base of the lobe. The posterior gyri 

 are the longest and broadest, and the anterior joins the convolution of the 

 under surface of the orbital lobule. 



B. The CONVOLUTIONS ON THE INNER SURFACE of the hemisphere 

 (fig. 62) are generally well defined ; but some being so long as to reach 

 beyond the extent of a lobe, the arrangement of them in lobes cannot be 

 followed, as on the exterior. 



Dissection. Without the use of a separate hardened hemisphere, the 

 parts now to be described will not be seen satisfactorily. If the student 

 possesses only one brain, he may bring into view much of the inner sur- 

 face by cutting of the left hemisphere as low as the white corpus callosum 

 in the median fissure. 



Convolution of the corpus callosum, gyrus fornicatus ( 18 ), is long and 

 simple, and arches round the body from which it takes its name. Begin- 

 ning at the base of the brain in the anterior perforated spot, it bends 

 backwards in contact with the corpus callosum (Cal), and below the back 

 of that body blends by a narrowed part with the uncinate convolution ( 19 ) 

 of the temporo-sphenoidal lobe. Anteriorly a sulcus separates it from the 

 following convolution ; and smaller gyri often connect the two across that 

 sulcus. 



The marginal convolution ( 17 ) is named from its position on th'e edge of 

 the median fissure. Its extent is rather more than half the length of the 

 hemisphere, for it begins in front at the anterior perforated spot, and ter- 

 minates near the back of the corpus callosum, just behind the fissure of 

 Rolando. It is much subdivided both internally and externally; and on 

 the under part of the frontal lobe (fig. 61) it lies internal to the olfactory 

 sulcus. Between it and the preceding convolution is situate the calloso- 

 marginal sulcus (i) which marks its hinder limit. 



The calloso-marginal sulcus (i, Huxley), designated from its situation, 

 begins in front below the corpus callosum, and ends behind, near the back 

 of the same body, by ascending to the edge of the hemisphere. Smaller 

 gyri uniting the two bounding convolutions, frequently interrupt it, and 

 secondary sulci are prolonged from it into the same convolutions. 



The quadrilateral lobule ( 18 ) reaches from the marginal convolution in 

 front to the parieto-occipital fissure behind. It is much divided by sulci, 

 and projects above to the edge of the hemisphere ; it joins below the gyrus 

 fornicatus. 



The occipitat lobule ( 2s ) is triangular in shape, with the base upwards, 

 at the margin of the hemisphere. Measuring about an inch and a half 

 in depth, it lies between the internal parieto-occipital fissure, PO, and the 

 calcarine sulcus (/). Sulci running from apex to base divide it into four 

 or five narrow convolutions. 



