335 
1888.] Dr A. Bruce on Absence of Coiyus Callosum. 
found to be a cyst 2 inches in' length and 1 in breadth, containing 
a serous fluid, and lined by a firm membrane. This formed roof of 
lateral ventricle or left side ; the body and most of posterior pillar 
of fornix were absent ; a portion of anterior column present. On 
velum interpositum was a small hydatid, and a considerable quan- 
tity of fluid in left and third ventricle. In the right ventricle every- 
thing was normal. Anterior commissure probably present ; middle 
abnormally thick. 
XXIX. Meierzejevski, Revue Anthropologies 1876, Xo. 17 ; see 
Onufrowicz, loc. cit, 313. — Corpus callosum thin, anterior commis- 
sure absent. 
XXX. Maclaren, Ed. Med. Jour., 1879. — Female, aged 32 ; imbe- 
cile, epileptic, deaf and dumb. Pia mater adherent along margins 
of longitudinal fissure ; convolutions thin ; white matter reduced ; 
ventricles greatly dilated ; septum lucidum absent ; c.c. represented 
by two narrow belts — one at posterior, one at anterior extremity. 
Body of fornix absent; anterior and posterior pillars represented. 
Anterior, middle, and posterior commissures intact. 
It is evident that the majority of the preceding cases are due to a 
primary arrest of growth, and are only to be properly interpreted by 
the study of the development of the cerebrum. We learn from the 
work, more especially of His and Mihalkovicz, that the anterior 
cerebral vesicle, which is primarily single, becomes at a very early 
period (about the eighteenth day) constricted in the middle line by 
the primitive falx cerebri, a process of vascular connective tissue. 
The two hemispheres thus formed grow up on either side of the 
falx, with their median walls at first plane and parallel to the latter ; 
but during the second month there appear on them two curved 
fissures almost concentric with the free margin of the hemisphere 
(fig. 31, from Mihalkovicz). These fissures are termed respectively 
the fissura hippocampi {f.li.) {ammons-furelie) and fissura choroidea 
{adergejiechts-furche). They begin anterior to the foramen of Monro, 
describe almost a semicircle over the corpora striata, and end near 
the tip of the temporo-sphenoidal lobe. The superior fissure forms 
a projection of the cerebral wall into the lateral ventricle, known as 
the pes hippocampi major, of which only the posterior part, that 
which projects into the posterior cornu, remains as a permanent 
structure. The inferior fissure, the fissura choroidea, is formed by 
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