vn] Limbic Lobe and Olfactory Area 183 



tectum or stria longitudinalis lateralis, may be seen, but the larger elevation of the indusium 

 forming the stria medialis is found nearer the middle line, and is that with which we are 

 concerned. 



A microscopic examination of these mesial striae is not devoid of interest ; we find that for 

 groundwork they have a band of longitudinally directed medullated nerve fibres of medium size, 

 upon which rests a strip of curious nerve cells. The presence of these cells was chronicled years 

 ago by Valentin, and they have since been described by Ramon y Cajal and others. In Nissl 

 specimens they appear as medium-sized pyramids with short processes, and they are arranged 

 in an irregular manner and poorly supplied with chromophilic elements ; serial sections prove 

 that posteriorly they can be traced round in continuation with the layer of pyramidal cells 

 in the dentate gyms'. The latter connection appears to be of some importance, because 

 one can think of no cells with which these can be homologous except those of the gyms 

 dentatus, and a certain resemblance in shape and staining reaction gives colour to the idea 

 that they form part of one original deposit of cell-elements, altered in position by the growth 

 and expansion of the corpus callosum. Followed in the frontal direction the cells of these 

 striae, after showing a gradual reduction in size, cease at about the middle of the corpus 

 callosum, but although the cells disappear, the band of fibres still continues and can be 

 plainly traced round into the septum lucidum, further than which I have made no attempt to 

 follow it 2 . It seems therefore that this forward extension represents the rudiments of the 

 gyms geniculi, just as the hinder part is the equivalent of the gyms supracallosus. 



PEDUNCLE OF THE CORPUS CALLOSUM GYRUS SUBCALLOSUS 



OF ZUCKERKANDL. 



Having completed our study of the cortex of the hippocampal portion of the " grande 

 lobe limbique" we will now turn to a consideration of the small field of substance into 

 which the mesial root of the olfactory nerve seems to penetrate, viz., the peduncle of the 

 corpus callosum, and finally we shall discuss the structure of the gyrus fornicatus. 



By the peduncle of the corpus callosum is meant that strip of substance which is 

 bounded above and behind by the rostrum, in front by the upper extremity of the shallow 

 tissura prima of His, and which broadening as it descends terminates at the base of the 

 brain, behind the anterior perforated space. Histologically its structure suggests that it is 

 an undeveloped area. 



In the part immediately underlying the rostrum a strongly developed zonal layer is 

 seen streaming out from the striae longitudinales mediales for a distance of 3 or 4 mm., 

 but in other parts of the surface this layer is feebly represented. 



The supraradiary layer is poor in nerve fibres. 



A line of Baillarger cannot be distinctly defined, but in the position which it usually 

 occupies, and also above and below it, there is a fair number of small or medium-sized 

 fibres apparently derived from the mesial olfactory root. 



1 Other observers (Giacomini, Golgi, Karnon y Cajal) have traced fibres pertaining to these striae to the fascia 

 deutata, but whether the hippocampus represents their origin or their termination is not known. The fibres of the 

 external striae also probably end in the dentate gyrus (Zuckerkandl, Bamon y Cajal). 



- Blumenau, Dejerine, and Ramon y Cajal give these fibres a similar distribution, and it seems not improbable that 

 they establish connections with the fibres of the inner olfactory root. 



