G94 THE NERVOUS SYSTEM 



pointed in section and is called the rostrum. The rostrum is connected to the 

 lamina cinerea by a thin layer of white substance, the basal white commissure. 

 The portion which intervenes between the splenium and rostrum, and which con- 

 stitutes the greater part of the corpus callosum, is called the body. The under 

 surface of the l)ody is adherent behind to the fornix, and in front to the sejitum 

 lucidum. In coronal sections (tigs. 415 and 428) it can be seen that the under surface 

 is free for some distance, except at its attachment in the middle line to the fornix 

 and septum lucidum. This free portion forms the roof of the lateral ventricles. 

 The upper surface remains free for a shorter distance than the lower, and ends at 

 the floor of the callosal fissure, where the fibres of the corpus callosum enter the 

 substance of the hemisphere. It forms in the transverse direction a gentle curve 

 with the concavity directed upwards. Viewed from above, the corpus callosum is 

 seen to present a transverse striation, this being an indication of the bundles of fibres 

 passing from hemisphere to hemis})here. These transverse striations are crossed 

 superticially by narrow longitudinal l)ands, the strise longitudinales mediales and 

 the strite longitudinales latcrales or ticnite tecta\ 



The striae longitudinales mediales are placed one on each side of the middle 

 line, and lie very close together, leaving a narrow interval between them which is 

 called the raphe. Traced forwards, they slightly diverge and pass round the genu 

 to the rostrum; here they separate more widely, and pass along the inner margin 

 of the anterior perforated space, under the name of the peduncles of the corpus 

 callosum, and are lost at the commencement of the fissure of Sylvius. Traced 

 1 »ackwards each medial stria is found to be continuous at the splenium of the corjjus 

 callosum with the fasciola cinerea. This turns round the splenium and is con- 

 tinuous beneath it with the fascia dentata. 



The taeniae tectae, or striae longitudinales laterales, lie on the u})per surface 

 of the corpus callosum under cover of the callosal gyrus. They also can be traced 

 backAvards round the splenium into continuity Avith the fasciola cinerea. The 

 medial and lateral striae together represent the free margin of the gray matter 

 on the inner surface of the brain, in the same Avay that the fascia dentata represents 

 it on the tentorial surface, and they constitute the remains of a degenerated supra- 

 callosal gyrus. 



The fibres of the corpus callosum after entering the cerebral substance spread out to every 

 ])art of the hemispheres (with the exception of small portions of the temporal lobes) in the 

 following sets : — 



The fibres from the genu pass forwards and outwards into the anterior paitof the frontal 

 lobe, and then sweep inwards forming the forceps minor. 



The fibres from the body have the foll(nviiig distribution : — Tlie uppermost fibres pass out- 

 wards and then upwards, and finally curve inwards to the upper and mesial surfaces of the fnmtal 

 and i)arietal lobes. The intermediate fibres pass transverselj' outwards to the same lobes. The 

 lowest fibres form the roof of the lateral ventricle, and are separated from that cavity only by 

 the epeudyma and epithelium, and divide into anterior and jjosterior sets which are differently 

 distributed. The anterior set ends in the opercula. The posterior set forms a layer called the 

 tapetum which follows the descending and jiosterior cornua of the lateral ventricle, roofing over 

 and bounding these cornua externallJ^ The part which follows the descending cornu is destined 

 i'or the temporal lohe. The portion which accompanies the posterior cornu passes to the under 

 part of the occipital lobe. 



The fibres from the splenium, and from the part which is folded below it, pass backwards and 

 outwards, and then sweep inwards forming the forceps major, the fibres of which pass to the 

 ]»osti'rior and ujjper parts of the occipital lobe. 



Dissection. — Having completed the examination of the corpus callosum and of the mesial 

 surface of the hemisphere, the student sliould next turn his attention to the lateral ventricles. 

 The remaining fibres of the body of tlu^ cor))US callosum should be divided a little to the right 

 of the middle line and reflected outwards. This dissection will expose the bo<1y and anteriorcornu 

 of the lateral ventricle. The posterior and descending cornua sliould next In: exposed by removing 

 their outer walls. 



The Lateral Vkntrtcles are cavities in the cerebral hemispheres Avhich are 

 continuous Avith the third ventricle and Avith one another, through the foramen 

 commune anterius (page 098). They are roofed over, as already cl escribed, by the 

 corpus callosum, and are lined throughout by epithelium, Avliich is reflected over the 

 choroid plexuses, and is not continuous Avith the lining of the third A-entricle except 

 at the foramina of Monro. This epithelial lining forms the ventricular A\all in 



