THE NERVOUS CENTRES. 121 



ment of the cerebellum. In consequence of these protrusions or outgrowths 

 taking place, the three primary cerebral vesicles are now converted into six 

 permanent rudiments of the brain and medulla oblongata. The anterior part 

 of the original anterior cerebral vesicle (fore-brain, prosencephalon), now divided 

 into two, constitutes the cerebral hemispheres, corpus callosum, corpora striata, 

 fornix, lateral ventricles, and olfactory bulbs. The hemispheres are at first rela- 

 tively small and do not conceal the parts formed from the middle primary vesicle 

 or the optic thalaini, which with the optic nerves, the third ventricle, and the 

 parts in its floor, are furnished by the posterior portion of the anterior vesicle 

 (inter-brain, thalamencephalon). By the third month, however, the hemispheres 

 have risen above the optic thalami, and by the sixth month above the cerebellum. 

 Fissures are seen on the surface of the hemispheres at the third month, but all 

 except one disappear. This one persists, and forms the fissure of Sylvius. The 

 permanent fissures for the convolutions do not form till about the seventh or 

 eighth month. The middle cerebral vesicle (mid-brain, mesencephalon) is at first 

 situated at the summit of the angle shown on Fig. 90. Its smooth surface is 

 soon divided, by a median and transverse groove, into four tubercles (tubercula 

 quadrigernina), which are gradually overlapped by the growth of the cerebral 

 hemispheres. Its cavity diminishes as its walls thicken, and contracts to form 

 the aqueduct of Sylvius. The crura cerebri are also formed from this vesicle. 

 The third primary cerebral vesicle at an early period (between the ninth and 

 twelfth week) consists of the hind-brain or metencephalon, forming the cerebel- 

 lum, pons Yarolii. and anterior part of the fourth ventricle, and of the after-brain 

 or myelencephalon, which forms the medulla oblongata with the rest of the fourth 

 ventricle. 



The development of the pituitary body has of late received much attention. It 

 is mainly formed by a diverticillum from the buccal involution of epiblast. At its 

 upper and front part this involution, from which the mouth or stomodaeum is 

 developed, forms a hollow saccular protrusion, which extends into the angle formed 

 by the bend of the fore -with the mid-brain. Here it comes in contact with a 

 median hollow protrusion, Avhich passes downward and backward from the 

 posterior portion of the anterior cerebral vesicle (Fig. 91, c, if). They become 

 intimately connected, and together form the pituitary body or hypophysis. 



When the medullary groove is first closed, the foetal spinal cord occupies 

 its whole length, and presents a large central canal, which gradually contracts in 

 consequence of the thickening and rapid growth of the epiblast around it. 

 This increase in thickness takes place principally at the sides, so that eventually 

 the central canal acquires on section the appearance of a slit. The two sides of 

 this slit eventually join in the middle, and the original canal is divided into two : 

 an anterior, which becomes the central permanent canal, which in after life is no 

 longer perceptible to the eye, though it is still visible on microscopic section ; 

 and a posterior, which becomes filled about the ninth week with a septum of 

 connective tissue from the pia mater, and forms the posterior fissure of the cord. 

 The anterior fissure is formed simply as a cleft left between the lateral halves 

 of the cord. 



After the fourth month the spinal column begins to grow in length more 

 rapidly than the medulla spinalis, so that the latter no longer occupies the whole 

 canal. The cord is composed at first entirely of uniform-looking cells, which soon 

 separate into two layers, the inner of which is composed of cells which increase 

 by division, and develop outgrowths which become axis-cylinders of nerve-fibres. 

 These cells are termed neuroblasts. The cells of the outer layer, known as 

 tpongioblagte, scatter themselves among the neuroblasts, forming the neuroglia 

 cells, some of them migrating inwards to form the ependymal lining of the cavi- 

 ties of the cord and brain. 



The cerebral and spinal membranes are, according to Kblliker, a production 

 from the protovertebral somites, and are recognizable about the sixth week. As 

 the fissures separating the segments of the cerebro-spinal axis appear, the mem- 



