THE STRUCTURE OF THE BRAIN STEM 409 



as the isthmus, the roof or dorsal wall forms the superior cerebellar 

 peduncles at the side, and between them a thin layer of nervous matter 

 known as the valve of Vieussens, or superior medullary velum. The 

 cavity of the third vesicle corresponds in the adult brain to what is 

 known as the fourth ventricle. 



The mesencephalon, or second cerebral vesicle, takes a relatively 

 small part in the formation of the adult human brain, though very 

 conspicuous in many of the lower types of brain. The whole of its wall 

 is transformed into nervous tissue, the roof or dorsal wall forming the 

 corpora quadrigemina, 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 aqueduct 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 vesicles are ultimately developed 

 the retina? of the eyes. By the development of nerve-cells in the optic 

 cup the ganglion-cell layer of the retina? 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 developed 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 hemispheres 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 thicken- 

 ing to form the nervous structures which occupy the ' interpeduncular 

 space ' at the base of the brain, viz. the posterior perforated spot, 

 the corpora mammilaria and the tuber cinereum. The roof of the 

 first cerebral vesicle remains thin and in its primitive epithelial condi- 

 tion, 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. 180). Their dorsal and lateral w^alls 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 



