304. Dll. G. ELLIOT SMITH ON 



of the evidence to be gained by a study of the cerebral commissures, it is of much greater 

 impoi'tance to investigate their constitution and morphology than to judge them on a 

 purely quantitative basis. 



In separating the two cerebral hemispheres the one from the other by means of a 

 mesial sagittal section, we cut throvigh the large white masses of the cerebral commis- 

 sures, which will be seen to consist of a small ovoid ventral commissure and a large 

 elongated dorsal commissure of a peculiar shajie. This dorsal commissure in Orycteropus 

 is composed of two divaricated limbs, which meet and fuse posteriorly. The larger dorsal 

 limb is the corpus callosum. Arching downward and backward from the ventral 

 surface of the caudal extremity of the coi'pus callosum, there is to be found a peculiarly 

 modified cortical area known as the hippocampus. The greater part of this peculiar 

 structure is submerged below the general level of the cortex, and its position is indicated 

 upon the surface by the hippocampal fissure. The cephalic margin of the hippocampus 

 is fringed by a band of white fibres — which is known either as fimbria or forma- — and 

 which is plainly visible upon the mesial surface of the hemisphere. 



After these preliminary remarks we are in a position to ajjpreciate the features of the 

 cerebral commissures. 



If we examine the mesial surface of the brain of Orycteropus which is exposed by a 

 mesial sagittal section (fig. 4, p. 291), we find the lamina terminalis extending obliquely 

 upward and forward from the oj)tic chiasma to reach the ventral or anterior commissure, 

 which consists of a large mass of transverse fibres presenting an oval outline in section 

 and measuring 4-5 mm. by 3"5 mm. Proin the dorsal aspect of the ventral commissure 

 the upper part of the lamina terminalis proceeds obliquely backward and uj)ward to the 

 inferior extremity of the ventral limb of the great bilaminar dorsal commissure. The 

 ventral limb of this dorsal commissure consists of fibres which are proceeding from 

 the hippocampus of one hemisphere through the fornix to the other hemisphere, and it 

 is custoniaiT to call these crossing fibres of the fornix the psaltcrimn *, from a fancied 

 resemblance in the arrangement of their homoloo-ues in man to a stringed instrument. 



The psalterium in Orycteropms consists of a large ventral fusiform mass of fibres, which 

 form the psalterium ventrale, and a slightly narrower upper part, the psalterium dorsale, 

 placed immediately below the dorsal limb of the great commissure, which latter is best 

 known by the designation corpus callosum. The psalterium dorsale joins the caudal 

 extremity of the corpus callosum, which consists of a large rounded mass of fibres 

 known as the spleninm. The corpus callosum is a larger structure than the psalterium, 

 and, unlike the latter, is of fairly uniform thickness, except at its cephalic and caudal 

 extremities, which are thicker than the rest of the commissure. It is placed obliquely, 

 and its posterior part is parallel to fhe psalterium dorsale; h\xt n^ the psalterium ventrale 

 l)ends downward toward the ventral or anterior commissure, a triangular interval is left 

 l^etAveen the j)salterium and corpus callosum. This interval is filled by a mass of grey 

 substance which will be subsequently described as the corpus pai-acommissurale, and 

 which in part represents the septum lucidiim of human anatomy. 



* FsnJtc)-iuin,^lifr(i, = coiniiiissur(t forn(cis.=^commis^Hrit hi/ipocaiii/ii. 



