4 APPENDIX. 



ferent organs seem to be formed, from the organization of some monstrous foetuses, 

 and from other considerations on which we cannot enter at this place, we are disposed 

 to conclude, that the spinal marrow is itself produced from the ramifications of the 

 sympathetic ganglia ; that the scmilunar ganglion is the first part of the nervous sys- 

 tem to assume an organized state ; and that the subordinate ganglia, the spinal marrow, 

 and lastly the brain, come successively into existence, and gradually arrive at their full 

 developement. 



It is not until about the third or fourth -week that a greyish-white fluid may be de- 

 tected in the cavities of the head and spine. From the fourth to the fifth -week the me- 

 dulla eblongata may be distinctly seen. It is then about twice as thick as the medulla 

 spinalisy which, before the developement of the limbs, is of an equal thickness through- 

 out its whole length, and presents a slight curvature near the commencement of the 

 medulla oblongata, owing to the flexion of the head upon the chest. The spinal marrow 

 at this time consists of two white strips of medullary matter, which offers a manifest de- 

 cussation at the place where it curves forwards at the margin of the inferior extremity 

 of the pyramids. It is not, however, the whole of the two cords of the marrow that 

 cross, but the middle or pyramidal fasciculi of each. The spinal marrow descends 

 from this point through the whole extent of the canal to the interior of the caudal pro- 

 longation. 



At the fifth -veek these stripes or chords form a junction of their interior and ante- 

 rior margins, a longitudinal gutter : their external and posterior magins are then full and 

 prominent. 



At the seventh iveek the spinal marrow is open throughout its whole length. On each 

 side of the fourth ventricle, a straight thin lamina is put forth, which inclines from with- 

 out inwards, applying itself to that of the opposite side, without, however, uniting with 

 it : these are the rudiments of the cerebellum, springing from the restiform bodies. The 

 cervical enlargement begins to appear, particularly its cephalic extremity. The for- 

 mation of the limbs coincides wit^h that of the corresponding enlargements of the chord. 



The longitudinal raphe, formed by the aproach of the interior margins of -the two 

 chords constituting the marrow, is continued upwards, and separates the tubercles, that 

 is, the laminse which represent them : the optic thalami are devolved. 



At the commencement of the third month, the marrow is still open at its superior half, 

 and extends to the extremity of the sacrum. The tubercula quadrigemina are volumin- 

 ous, hollow and separated by the median furrow : the optic thalami are full. The two 

 cervical and lumbar prominences are a third of a line thicker than the body of the mar- 

 row: Tiedemann has not observed the junctions of its exterior margins until the end 

 of the third month. Serres has seen it a fortnight earlier. This junction takes place 

 from beneath upwards. 



At twelve -weeks the marrow extends only to the middle of the sacrum. The tubercula 

 quadrigemnia are united and form a canal ; at this period the mammillary eminences 

 and the corpora striata may be seen. The internal canal, which is now formed by the 

 junction of the margins of the marrow communicates with the fourth ventricle. Ac- 

 cording to M. Desmoulins, this canal results from a sinus, formed by the fold of the pia 

 mater as it dips into the interior of the marrow, or rather between both the cords of 

 which the marrow is then formed. The precise period at which this canal is com- 

 pletely obliterated, has not been ascertained. M. Carus conceives that the pectoral 

 portion of the marrow is the first to close, and that the canal is obliterated along its 

 whole extent, owing to the formation of the gray substance. 



At the fourth month, the spinal marrow reaches only to the base of the sacrum; the 

 cervical eminence is larger than the lumbar. The two contiguous cords of the marrow 

 divide, in the medulla oblongata, each into three others much smaller. The internal 

 or pyramidal forms a tolerably broad surface, as in fishes and reptiles, and evidently 

 crosses, as was already noticed, with that of the other side about the fourth or fifth 

 week of the foetal life. The middle cord, or the corpus oblivare, is placed above the 

 former; some of its fibres ascend to the tubercula quadrigemitui, and unite with those 

 proceeding 1 from the opposite side, in order to form the vault of the aqueduct of Syl- 

 vius. The external or restiform cord, proceeding from the lateral and posterior por- 

 tion of the marrow, forms the prominent paries of the fourth ventricle, and advances 

 into the cerebellum. At this period the annular protuberance is perceptible. The 

 interior canal of the marrow is now very narrow, and still communicates with the fourth 

 ventricle. It is not until towards the end of the fourth month that the lumbar and sa- 

 cral nerves become elongated, and form what has been improperly called the caiirla 

 equina, which at first does not exist. The pia mater which penetrates by the posterior 

 median furrow, is observable in the centre of the marrow. 



