01? THE CEREBRO-S FINAL SYSTEM. 5 



At thejifth mnnth, the pyramidal eminences are evident : there still exists a commu- 

 nication between the fourth ventricle and the canal of the marrow. The two swellings 

 of the cord are well marked. The annular protuberance becomes more distinct, and 

 the corpora siriata are large. The increased thickness of the tubercles has narrowed 

 considerably the cavity which they formed by their approach. The marrow extends at 

 this period no farther than the margin of the fifth lumbar vertebra. 



The human embryo possesses a caudal prolongation until the fourth month of uterine 

 life. At this period it disappears, and its disappearance coincides with the ascension of 

 the spinal marrow in the vertebral canal. If the ascension of the marrow is arrested, 

 the human fetus is born with a tail, as has been observed in several cases. The cir- 

 cumstance of the spinal marrow descending 1 lower in the vertebral canal the younger the 

 foetus, has attracted the particular notice of those physiologists, to whose researches we 

 are indebted for our knowledge of the subject under consideration. M. Tiedemann, 

 who offers the most rational explanation of this phenomenon, supposes that the marrow 

 descends not so far in the canal of the full-grown fetus, as in that of the early embryo, 

 because the vertebral column grows more rapidly in length, than the nervous Cord 

 tvhich it is destined to protect. 



To wards the end of the sixth month, the corpora vlivaria form a well-marked lateral 

 projection. At this epoch may be seen the internal and middle cords, forming the pe- 

 duncles of the brain, plunging into the optic thalami which they formed by their en- 

 largement. The fibres composing them may be perceived on scraping away a thick 

 pulpy layer from their interior and superior aspect. A few fibrous portions detach 

 themselves from their internal side, and proceed outwards to the mammillary eminence. 

 All the other fibres continue to advance from behind forwards, and from within out- 

 wards, beneath -the corpora striata, and proceed, in a diverging form, to the cerebral 

 lobes. A few fibres may be seen entering them. In the course of the sixth month 

 the transverse furrow separating the eminentia quadrig'endna begins to appear, or rather, 

 each of these prominences becomes more developed. 



At the seventh month, the length of the marrow is nearly the same. The transverse 

 fibres which compose the annular protuberance are now distinct, and they may be seen 

 interlacing with those of the pyramids. This part results from the following disposi- 

 tion ; the fibres of one lateral hemisphere of the cerebellum are continued beneath the 

 spinal marrow with the fibres of the opposite hemisphere, by layers which alternate 

 with the planes of fibres proceeding obliquely from the pyramids to the optic thalami. 



At the eighth month the marrow reaches only to the fourth lumbar vertebra, and at 

 the ninth month it is at the margin of the third. The interior canal of the marrow still 

 exists, and remains until from six months to a year after birth. It is at the last months of 

 gestation that the disposition of the medullary fibres of the marrow may be most dis- 

 clistinctly traced, and the mode of formation of the mesocephalon or pans Varolii, 

 which is only a continuation of the marrow, becomes most evident. 



All the white or medullary parts which are seen at the base of the brain, manifestly 

 arise from the superior part of the spinal marrow. (OLLIVIEH. sur'le Beveloppemeitf d& 

 la Moelle Opiniere.) 



II. Of the Formatioji of the Brain. -The. interesting results of Dr. Tiedemann's re- 

 searches on this subject may be reduced to the following heads : 



" 1. In the commencement of pregnancy, especially about the second month, the 

 earliest period at which the brain can be rendered perceptible by the action of alcohol, 

 this organ is very small in proportion to the spinal marrow. In fact, it results from the 

 prolongation upward and forward, of the two principal cords, the olivary and pyrami- 

 dal. All its superior part is open, or, more properly speaking, forms a broad gutter, 

 which at once comprehends the third ventricle, the aqueductus sylvii, the fourth ven- 

 tricle, and calamus scriptorius. This gutter is interruptedly continuous with the canal, 

 which traverses the whole length of the marrow. 



'* 2. The cerebellum evidently originates from the spinal marrow ; from the lateral 

 parts of which arises, on each side, a small flattened cord. These two, at first so dis- 

 tinct and separate that they may be readily parted without laceration, afterwards unite 

 so as to form ttie roof of the fourth ventricle. Then only the brain, viewed from above, 

 ceases to represent a gutter ; and the lamina: and branches of the cerebellum are formed 

 at a much later period. 



"3. The mass which supports the tubercula quadrigemina equally shows itself in its 

 origin, under the form of two small thin membranes, which arise from the olivary cords 



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