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ME. W. H. FLOWER ON THE CEREBRAL COMMISSURES 
The transverse section (Plate XXXVII. fig. 6) shows the corpus callosum curving up 
at the outer extremities owing to the upward development of the lateral ventricles, as 
in the rabbit, and in the foetal condition of the higher mammals. The corpora striata 
(K, K) are very large. The anterior commissure exceeds in vertical depth the corpus 
callosum. The septum, broad below where it rests on the anterior commissure, 
diminishes above to a narrow edge, where it touches the under surface of the corpus 
callosum; but there is no part which can properly be called septum lucidum. On 
each side of the middle line are seen the vertical white fibres, forming the commence- 
ment of the columns of the fornix. 
Plate XXXVII. figs. 7 & 8 are taken from the brain of the Common Hedgehog 
(Erinaceus europceus). The transition from the Sloth’s brain to this is easy, although it 
presents a wide difference from that of the Eabbit. The inner surface of the cerebrum 
shows no trace of any sulcus, except the deep one of the hippocampus (Q), which is 
placed very near the hinder border of the truncated hemisphere, and terminates a little 
way behind and below the posterior end of the corpus callosum. The last named body 
is extremely reduced in size, its length being but one fifth that of the entire hemisphere. 
Its obliquity is so much increased that its general direction is rather vertical than hori- 
zontal. The psalterial fibres form a distinct projection (N) in the section closer to the 
body of the corpus callosum than in the two previously described brains. The septal 
area is much reduced, and the anterior commissure increased in bulk. The great size of 
the olfactory ganglion is very remarkable. 
The transverse section shows a corresponding simplicity, and agrees in all its essential 
characters with that of the Sloth. The oblique position of the corpus callosum gives its 
section an apparent thickness, which it would not possess if divided, as in the higher 
mammals, at a right angle to the plane of its upper surface. 
These are examples of some of the modifications of the commissural apparatus of the 
cerebral hemispheres among the placental mammals. They might be considerably multi- 
plied, but they are sufficient for the purpose of affording a basis of comparison with the 
same parts in the Marsupials and Monotremes. 
Before entering upon this part of the subject, it may be desirable to give an outline of 
the present condition of knowledge upon it. A reference to the works of comparative 
anatomists who wrote before the year 1887, shows that up to that period no important 
distinction had been suspected to exist in the cerebral organization of the placental and 
the implacental mammals. In the Philosophical Transactions of that year, however, 
appeared the memoir of Professor Owen “ On the Structure of the Brain in Marsupial 
Animals,” in which was announced the absence in these animals, of the “ corpus callo- 
sum and septum lucidum.” A transverse commissure between the hemispheres superior 
to the anterior commissure is described, but called by Professor Owen “fornix” or 
“ hippocampal commissure.” Of this it is stated, “ This commissure may, nevertheless, 
be regarded as representing, besides the fornix, the rudimental commencement of the 
