1888 .] Dr A. Bruce on Absence of Cor^pus Callosum. 
337 
causes the intercepted portion of the primitive falx to atrophy 
(fig. 33), so that the falx {fix) and tela choroidea superior {tel. clior.) 
become apparently two quite independent structures. 
The portion of the marginal arches behind the point of fusion 
gives origin to the fornix {fornixf the fascia dentata {fasc. dent), 
and the nervus Lancisii (l7ic). On its outer border is the fissura 
hippocampi {f.li.) proper ; while the anterior part of this fissure 
now lying above the corpus callosum becomes the callosal sulcus 
(see Milhalkovicz, Geschichte des Gehirns,'^^. 120-130). 
If we apply these facts to the study of the recorded cases of 
absence or partial defect of the corpus callosum, we find that the 
majority of these cases can be explained on the hypothesis of 
arrest of development, and that they may be classified according to 
the period at which this arrest takes place, the appearance of the 
brain varying accordingly. 
1. The Falx may constrict the Anterior Cerebral Vesicle, either 
not at all, or insuficiently. — (Lesion occurs during first three weeks.) 
The cerebrum will consist of a single vesicle, or of two imperfectly 
divided hemispheres, united by an unthinned septum (or grey 
matter). There will be one ventricular chamber, no tela choroidea 
superior, no convolution of the marginal arch, and therefore no 
fornix, no anterior commissure, and no corpus callosum. See cases 
recorded by Turner, Journal of Anatomy and Physiology, xii. 
p. 241 (fig. 29) ; Bianchi, Storica del Monstri del Duo Corpi, 
Torino, 1749, p. 100; Forster, Missbildungen des Menschen, 1861, 
p. 87, cases of Cyclopia; Hadlich, Arch. f. Psychiatric, x. p. 99 
(figs. 26, 26a, 27, 27a) ; and Wille, same Number, p.^597 (fig. 28). 
2. The two Hemispheres perfectly divided, hut Septum Lucidum 
and Marginal Arch, if developed, fail to unite. — There will be no 
anterior commissure, no corpus callosum, no psalterium of fornix. 
Tela choroidea superior continuous with falx cerebri. (Fornix 
present if marginal arch developed.) Development arrested before 
the fourth month. Cases II. Ward, III. (?), IV. (?) Foerg, XL, XV. 
3. Hemispheres formed, hut Septa Lucida miited only hy Antero- 
inferior Angle — Anterior commissure present. Other structures as 
in Class II. (Development arrested during fourth month.) Cases 
(several imperfectly recorded) I., V., VI., VII. (?), VIII., IX., 
X.- (?), XII., XIIL, and my case. 
