Fig. 258. Brain of a three months’ human embryo: natural size. (From Kölliker.)
1. From above with the dorsal part of hemispheres and mid-brain removed; 2. From below. f. anterior part of cut wall of the hemisphere; f´. cornu ammonis; tho. optic thalamus; cst. corpus striatum; to. optic tract; cm. corpora mammillaria; p. pons Varolii.
The cerebral hemispheres undergo in Mammalia the most complicated development. The primitive unpaired cerebral rudiment becomes, as in lower Vertebrates, bilobed, and at the same time divided by the ingrowth of a septum of connective tissue into two distinct hemispheres ([figs. 260] and [261] f and [258] 1). From this septum is formed the falx cerebri and other parts.
The hemispheres contain at first very large cavities, communicating by a wide foramen of Munro with the third ventricle ([fig. 260]). They grow rapidly in size, and extend, especially backwards, and gradually cover the thalamencephalon and the mid-brain ([fig. 258] 1, f). The foramen of Munro becomes very much narrowed and reduced to a mere slit.
The walls are originally nearly uniformly thick, but the floor becomes thickened on each side, and gives rise to the corpus striatum ([figs. 260] and [261] st). The corpus striatum projects upwards into each lateral ventricle, giving to it a somewhat semilunar form, the two horns of which constitute the permanent anterior and descending cornua of the lateral ventricles ([fig. 262] st).
Fig. 259. Transverse section through the brain of a rabbit of five centimetres. (After Mihalkovics.)
The section passes through nearly the posterior border of the septum lucidum, immediately in front of the foramen of Munro.
hms. cerebral hemispheres; cal. corpus callosum; amm. cornu ammonis (hippocampus major); cms. superior commissure of the cornua ammonis; spt. septum lucidum; frx 2. vertical fibres of the fornix; cma. anterior commissure; trm. lamina terminalis; str. corpus striatum; ltf. nucleus lenticularis of corpus striatum; vtr 1. lateral ventricle; vtr 3. third ventricle; ipl. slit between cerebral hemispheres.
With the further growth of the hemisphere the corpus striatum loses its primitive relations to the descending cornu. The reduction in size of the foramen of Munro above mentioned is, to a large extent, caused by the growth of the corpora striata.