1000 DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM. 



DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM. 



Upon each side of the fore-brain vesicle, prosencephalon, which is covered 

 externally by epiblast and internally by ependyma, there grows a large pe- 

 dunculated hollow vesicle, the rudimentary cerebral hemisphere, telencephalon 

 (the forerunner of the rhinencephalon, pallium and corpus striatum). The 

 relatively narrow opening in the pedicle is the rudimentary foramen of Monro. The 

 small middle portion behind the two hemispheres is the interbrain, diencephalon 

 (the forerunner of the thalamus, together with the metathalamus and the epithal- 

 amus). The interior of this contains the third ventricle, which about the second 

 month becomes elongated toward the base in the shape of a funnel as the tuber 

 cinereum, with the infundibulum. The thalami, developing on each side from 

 the floor of the interbrain, reduce the foramen of Monro to a semilunar cleft. 



In the second month the corpora albicantia also develop at the base, in the 

 third month the chiasm; the commissures are formed within the third ventricle, 

 in the third month. The hypophysis, belonging to the mid-brain, is a diverticulum 

 of the pharyngeal mucosa through the base of the cranium toward the hollow 

 infundibulum, which grows to meet it, and which subsequently become constricted 

 off. There is thus an effort at union between the cavity of the fore-gut and the 

 medullary canal. It should, further, be mentioned here that in the amphioxus, 

 the goose, some parrots and the lizard, the medullary canal originally communicates 

 with the rudimentary hind-gut by means of a passage-way (myeloenteric canal) . 

 The choroid plexus, which grows into the cavity of the hemispheres by way of 

 the foramen of Monro, is a vascular hyperplasia of the ependyma. In the fourth 

 month the conarium develops, and at this time the quadrigeminal bodies are 

 already covered by the hemispheres. Within the cavity of the hemisphere 

 there develops in the second month the striate body, in the fourth month the 

 cornu Ammonis. In the third month there develops the fossa of Sylvius, at the 

 bottom of which the insula is formed as a part of the original trunk of the fore- 

 brain and over which, at the end of fetal life, the operculum projects. From 

 the seventh month the remaining convolutions of the brain are formed. Medul- 

 lated fibers are already present in the cortex of the newborn in the central con- 

 volutions, as well as the paracentral lobule. Finally, in the third month such 

 fibers appear in some regions of the frontal and parieto-temporal lobes. 



The mid-brain vesicle, mesencephalon (rudiment of the quadrigeminal bodies 

 and cerebral peduncles) becomes gradually covered by the growth backward of 

 the hemispheres; its cavity becomes reduced to the aqueduct of Sylvius. The 

 surface of the vesicle becomes divided into four parts, the quadrigeminal bodies; 

 a longitudinal groove appearing in the third month, and a transverse groove in 

 the seventh month. The cerebral peduncles are formed on the floor as thicken- 

 ings. 



From the hind-brain, metencephalon (rudiment of the pons and cerebellum), 

 there develop separately the hemispheres of the cerebellum, which, growing back- 

 ward, unite in the middle line. In the sixth month the hemispheres are more fully 

 developed, and the vermiform process is formed. The cerebellum covers the sub- 

 jacent unclosed portion of the medullary tube down to the calamus. The open- 

 ing of the medullary tube at the calamus further the tendency of the third 

 ventricle to communicate with the pharynx, facilitates an understanding of the 

 structure of articulates, in which the mouth traverses the central nervous system, 

 and the latter passes down on the ventral aspect. The pons is formed on the floor 

 of the hind-brain in the third month. 



The spindle-shaped after-brain, myelencephalon , grows narrower in its course 

 downward and becomes the oblongata, the upper portion of which exhibits the 

 open medullary cavity. 



From the medullary canal, downward from the after-brain, the spinal cord 



levelops, the gray substance nearest the cavity; later, this becomes surrounded 



by the newly formed white matter. The ganglion-cells (amphibia) increase by 



division. At first the spinal cord extends to the coccyx. As in adults the extremity 



the spinal cord extends only to the first or second lumbar vertebra, the spinal 



rd does not keep pace in growth with the spinal column; and in consequence 



the lower spinal nerves must undergo increase in length. The extent to which 



disparity in the growth of the spinal column and the spinal cord respectively 



that, tor instance, the former grows too rapidly, or the latter too slowly, may 



produce sensory derangement or paralysis in the lower extremities in children 



