1881.] Comparative Structure of the Brain in Rodents, 19 



In my former memoir I particularised the same features as notice- 

 able in the brain of the sheep and pig, viz., the existence of an 

 extensive area of the five-laminated rich ganglionic cortex in the 

 anterior regions of the hemisphere with two projecting arms of a 

 similar formation extending backwards. These latter — the Sagittal 

 and Sylvian groups — embrace betwixt them an intermediate region 

 of the parietal lobe characterised by a six-laminated cortex with 

 solitary arrangement of the ganglionic cells which are few and poorly 

 developed. 



Central Projections of Olfactory Medulla. — The medullary connexions 

 between the olfactory bulb and lobe and the cortex of the rest of the 

 cerebrum consist of the following series : — 



1. A central decussating and commissural fasciculus. 



2. Medullated connexion with striate body. 



3. Arciform medulla to limbic lobe and occipital pole. 



4. Superficial olfactory fasciculus, the so-called " outer olfactory 

 root." 



1. Central Olfactory Fasciculus. — Of the various medullated con- 

 nexions of the olfactory organ this exhibits the most interesting and 

 novel features. After passing back from the olfactory bulb to its 

 decussation in the anterior commissure, its posterior projections 

 traverse the substance of the corpus striatum, and in this course its 

 strands are widely separated by the passage through them of numerous 

 fasciculi of medullated fibres which, arising behind run forwards, 

 some blending with the other fibres of the olfactory fasciculus, but 

 most traversing its structure at right angles so as to emerge in front 

 of it as a delicate fibrous dissepiment betwixt the corpus striatum 

 proper and that portion lying in the olfactory area. The posterior 

 projection of the central fasciculus upon reaching the outer boundary 

 of the striate body takes a different course in the rat and the rabbit. 

 In the former it divides here into numerous secondary branches which 

 are directed backwards towards the occipital pole of the hemi- 

 sphere to be distributed to the cortex described here as of the modified 

 lower limbic type. A limited number of fibres pass outside the striate 

 body. In the rabbit, on the other hand, this fasciculus turns upwards 

 upon the outer surface of the corpus striatum, and reaching the upper 

 pole of this ganglion suddenly divides into a brush-like head of fibres, 

 which decussating and interweaving with the callosal and projection 

 systems here eventually terminate in the cortex, of the vertex at its 

 marginal angle (along the great longitudinal fissure). 



2. Connexions with Striate Body. — Medullated bundles pass from 

 the medulla of the olfactory lobe and the granule layer of the bulb 

 backwards into the basal portion of the caudate nucleus separated 

 from each other in this region by oblong grey masses which enclose 

 nerve-corpuscles. The nuclear masses are traversed throughout by 



c 2 



