OF THE CEEEBETBI OF THE QHAHETJMAHA. 
189 
into the substance of the hemisphere. It must be observed that in the human brain there 
is some variation in this respect, but the average depth of the calcarine fissure is well 
illustrated in this example, as also the usual size of the posterior cornu. This cavity 
appears in section as a mere fissure curved round a delicate stratum of white substance, 
which covers internally the grey matter of the calcarine sulcus, and which constitutes 
the ventricular surface of the hippocampus minor. In section C no trace of the opening 
can be discovered. 
Such is the appearance presented by this portion of the brain, in the highest state of 
perfection which that organ attains ; a condition, however, always the least favourable 
for studying the morphological significance and true relations of its several parts. The 
original design, now obscured by special adaptive modifications, can only be traced 
either by observing the gradual evolution of the same parts from their most rudimentary 
condition, or by a comparison of similar structures in other animals of simpler organiza- 
tion. 
By the first method we learn that while the hemispheres of the brain are mere sacs, 
and perfectly devoid of convolutions, the calcarine is one of the first of the sulci which 
appears on the surface (about the fifteenth week, according to Tiedemaniv), and coinci- 
dent with it, an elevation is seen upon the interior of the ventricle, the future hippo- 
campus minor. The eminence is, in fact, formed simply by an involution of the wall of 
the original ventricular cavity, and such portion of this cavity, situated in the posterior 
lobe, as has escaped being closed in by the growth of the surrounding cerebral sub- 
stance, constitutes the posterior cornu. The variability of its extent in the human sub- 
ject, and consequent apparent variability of the hippocampus minor, is well known 
Little physiological importance can, however, be attached to the size of the latter as 
commonly estimated by the projection into the ventricle. The real amount of cortical 
or ganglionic neurine surrounding the calcarine sulcus can only be ascertained by an 
examination of the length, depth, and complexity of that sulcus, and remains unaltered 
whatever may be the extent to which the cavity of the ventricle is closed : just as the 
size and form of the corpus striatum would be unaffected by the absence or closure of 
the portion of the lateral ventricle which lies in contact with it. Such an examination, 
both in different individuals of the human race, and in the brains of various animals, 
may supply important data in future investigations concerning the functions of special 
portions of the grey matter on the surface of the cerebrum. 
* Many of the variations in the condition of these parts must be ascribed rather to pathological changes 
than to original conformation. Thus, in aged and debilitated subjects, the posterior cornu is often enlarged, 
and somewhat funnel-shaped, the calcarine projection being also more or less obliterated, a circumstance 
arising apparently from gravitation of the intra-ventricular fluid during long continuance of the recumbent 
posture. In atrophy of the cerebral substance, from whatever cause, attended by an increased size of the 
ventricular cavities, the change is usually most strikingly seen in the posterior cornu. , 
