292 DEVELOPMENT OF THE CEEEBRO-SPINAL AXIS. 



be no doubt that every animal function, from the automatic motions of 

 the obscurest living form up to processes of intellection of man, depend 

 upon this structure as on an instrument, we may, by a due comparison of 

 the habits, instincts, or other phenomena in such cases with the existing 

 nervous development, arrive at 'true conclusions of the connection between 

 its structure and its functions. :. We shall therefore indicate, in a general 

 manner, the order of development of this system in man, and then its 

 permanent stages in the animaL series. 



The nervous system first n:i.kes its appearance in the serous lamina 

 Course of de- of the germinal membrane and in the midst of the pellucid 

 veiopment of area ag ^ e p r j m itiy e trace, a delicate and pale- white line ris- 

 ous system, ing somewhat above the general surface of the germinal area. 

 This line soon presents a conical' aspect ; the thicker portion is destined 

 to become the head of the embryo. After a short interval, the membrane 

 is gathered into a fold on each side of the primitive trace, and these folds, 

 advancing toward each other, constitute the dorsal laminse, which, when 

 their edges have met and coalesced, form a tubular cavity a. rudimentary 

 preparation for the vertebral column. Beneath the tube so arising may 

 be discovered, at this stage, a line of nucleated cells the chorda dorsalis. 

 As the edges of the dorsal laminae approach each other, they assume a wavy 

 form, and simultaneously a bending forward or curvature of the embryo 

 occurs, so that the vertebral tube becomes arched. In the middle wavy 

 portion are now to be seen rectangular plates, the elements of the future 

 vertebras. The coalescence of the middle part of the dorsal laminas 

 takes place first, the ends as yet diverging in the portions which corre- 

 spond respectively to the head and the sacrum. The spinal marrow and 

 the brain thus arise at the primitive trace, the brain being a superposed 

 or additional structure to the spinal marrow ; for now the wavy edges of 

 the anterior extremity are gradually seen to give origin to three cells by 

 their juxtaposition : 1st. The epencephalpn, a single cell, to produce the 

 medulla oblongata : its' cavity is to be the fourth ventricle ; 2d. The mes- 

 encephalon, also a single cell, for the corpora quadrigemina : its cavity 

 is to be the ventricle of Sylvius ; 3d. The. deutencephalon, a single cell, 

 for the optic thalami : its cavity is to be the third ventricle. Though at 

 first transparent and fluid, the nervous matter becomes by degrees more 

 consistent and covered over with a thin layer of membrane, the indica- 

 tion of its future investitures. The rudiment of an eye, under the form 

 of a protrusion, now appears from the most anterior cell ; and in like 

 manner the auditory apparatus emerges from the cell of the medulla ob- 

 longata, from the anterior part of which, by the coalescence of a pair of 

 fasciculi which have arisen, the cerebellum begins to form. At this peri- 

 od, through the continued curvature of the embryo, the cell of the cor- 

 pora quadrigemina has become most anterior. 



