THE STRUCTURE OF THE BRAIN STEM 



into nervous tissue, the roof or dorsal wall forming the corpora quadritteiniin 

 while the two crura cerebri are developed in its ventral wall. The cavity of 

 the second cerebral vesicle is retained as a narrow canal known as the aque- 

 duct of Sylvius, and connects the fourth ventricle with the third ventricle 



Very soon after its first appearance the first cerebral vesicle is modified 

 by the formation of lateral expansions, known as the optic vesicles, which 

 later on are constricted off from the central part of the cavity so as to be 

 connected with this by two short tubular passages, the optic stalks. From 

 the optic vessels are ultimately developed the retina? of the eyes. By the 

 development of nerve-cells in the optic cup the ganglion-cell layer of the 

 retinae is produced, and from these cells fibres grow back along the optic 

 stalk and make connection with the grey matter developed in the lateral 

 wall of the fore-brain and with the adjacent parts of the mid-brain, viz. the 

 superior corpora quadrigemina. The large masses of nervous tissue de- 

 veloped in the lateral walls of the fore-brain are the optic thalami, which 

 represent the head ganglia of the brain stem. The front portion of the first 

 cerebral vesicle expands in a forward and downward direction, and from the 

 upper and lateral aspects of the outgrowth thus formed the cerebral hemi- 

 spheres are produced as two hollow pouches. The original back part of the 

 fore-brain is sometimes spoken of as the diencephalon, while the anterior 

 part of the cerebral hemisphere growing from it is the telencephalon. The 

 floor or ventral wall of the fore-brain undergoes moderate thickening to form 

 the nervous structures which occupy the ' interpeduncular space ' at the 

 base of the brain, viz. the posterior perforated spot, the corpora mammillaria 

 and the tuber cinereum. The roof of the first cerebral vesicle remains thin 

 and in its primitive epithelial condition, like the roof of the back part of the 

 fourth ventricle. 



In the course of development the cerebral hemispheres become larger 

 than the whole of the rest of the brain put together, growing backwards 

 over the latter as far as the middle of the cerebellum (Fig. 179). Their 

 dorsal and lateral walls become much thickened and consist of white matter 

 internally and grey matter externally. The part of the hemisphere which 

 lies over the first cerebral vesicle is undifferentiated and remains as a simple 

 epithelial layer. This becomes closely applied to the similar layer forming 

 the roof of the third ventricle, from which it is separated only by a process 

 of the pia mater carrying numerous blood-vessels (the velum interpositum). 

 In the adult brain the cavities of the cerebral hemispheres are known as the 

 lateral ventricles, the remains of the first cerebral vesicle receiving the name 

 of the third ventricle. The lower and outer part of the hemispheres, i.e. 

 the part which is first formed, becomes much thickened and forms the corpus 

 striatum, which is closely applied to the front and outer part of the optic 

 thalamus. In the corpus striatum two masses of grey matter are developed, 

 namely, the nucleus caudatus and the nucleus lenticularis. A layer of nerve 

 fibres ascends from the brain stem to be distributed throughout the whole 

 of the cerebral hemispheres. This forms a sort of capsule to the optic 

 thalamus, lying between this body and the corpus striatum behind, but in 



