THE BEAIN. 521 



remains for some time undivided. Early in tlie iifth montli a 

 median longitudinal groove is formed along its anterior part, and 

 shortly afterwards a pair of transverse^ grooves appear, dividing 

 the roof into a larger anterior, and a smaller posterior division. 

 The median groove, which divides the posterior part into right 

 and left lobes, is not completed until the seventh month. 



In connection with the floor of the mid-brain the crura 

 cerebri are formed, as a pair of /thick bundles of longitudinal 

 nerve fibres. 



e. The Cerebellum. 



The general history of the cerebellum has already been given. 

 The surface remains smooth until the end of the third month. 

 During the fourth month the convolutions and sulci appear, and 

 rapidly increase in number and in importance. From the fourth 

 month onwards the lateral lobes grow rapidly, and at the same 

 time the transverse fibres of the pons Varolii are developed. 



f. The Medulla Oblongata. 



The roof of the medulla oblongata is wide and thin, almost 

 from the first. The floor, along the actual median line, is also 

 thin ; the sides are greatly thickened, and are divided by well- 

 marked grooves along their inner surfaces (cf. Fig. 228) into 

 ventro-lateral and dorso-lateral areas. 



It has been recently pointed out that a similar division may 

 be recognised in the side walls of the more anteriorly situated 

 portions of the brain, the ventro-lateral areas forming the ventral 

 half of the brain as far forwards as the optic chiasma ; while the 

 cerebellum, the optic lobes, and the whole of the cerebral 

 hemispheres belong to the dorso-lateral areas. It is uncertain 

 as yet whether this distinction is of any real morphological 

 importance. 



2. The Spinal Cord and Spinal Nerves. 



The histological development of the spinal cord and nerves 

 in human embryos has been studied in considerable detail, more 

 especially by Professor His ; and it is on his descriptions that 

 the following account is mainly based. 



The spinal cord, in the early stages of its development, is 

 merely a specialised tract of epithelium. Some of the com- 



