398 THE BRAIN 



It separates the orbital part of the inferior surface from the 

 medial surface. 



The medial occipital border can be seen from below, after 

 the hind-brain and the mid-brain have been cut away from 

 the cerebrum, or from behind, when the posterior parts of 

 the hemispheres are separated from one another. It extends 

 from the occipital pole to the posterior end of the corpus 

 callosum. It lies along the margin of the inferior part of 

 the posterior portion of the longitudinal fissure, in relation 

 with the wall of the straight sinus; and it separates the medial 

 surface from the posterior part of the inferior surface. 



Fissura Longitudinalis. The longitudinal fissure is the 

 great median cleft between the two cerebral hemispheres ; 

 anteriorly and posteriorly, it completely separates the hemi- 

 spheres from each other, but the intermediate part is 

 bounded below by the corpus callosum which passes between 

 the hemispheres and connects them together. If the two 

 sides of the longitudinal fissure are gently drawn asunder, 

 the upper surface of the corpus callosum will be displayed. 

 When the brain is in situ the longitudinal fissure contains 

 the falx cerebri of the dura mater (p. 104), a fold of arachnoid, 

 the pia mater covering the medial surfaces of the hemispheres, 

 the anterior cerebral arteries and veins, with their branches 

 and tributaries. The falx cerebri was removed when the 

 brain was taken from the skull ; the other membranes and 

 the vessels are still in situ, and they should not be disturbed 

 till the medial surfaces of the hemispheres can be examined 

 (p. 419). 



Dissection. Separate the cerebellum, pons and medulla from 

 the cerebrum, if that has not already been done, by cutting 

 transversely through the upper part of the mid-brain. Then, 

 if two brains are available, split one of them in the median 

 plane by placing a long knife in the longitudinal fissure and 

 dividing carefully the various parts which connect the two 

 halves together. All three surfaces of each hemisphere will 

 then be exposed, the gyri and sulci can be studied fully and 

 satisfactorily, and the terminal parts of the anterior and posterior 

 cerebral arteries (pp. 384, 386) can be examined. If only one 

 brain is at the disposal of the dissectors they should not, at this 

 stage, separate the cerebral hemispheres from each other, but 

 should endeavour to follow out the gyri and sulci with the 

 various parts of the brain in position. No doubt they will 

 study the hemispheres in that way at some disadvantage, but 

 as the dissection proceeds, opportunities will occur which will 

 enable them to examine those districts of the surface which 

 they can see only imperfectly? a*t present. 



' 



