HUMAN AND MAMMALIAN. J,T 



CHAPTEE IV. 



THE PASSAGE INTO THE MEDULLA OP THE POSTERIOR VESICULAR COLUMNS 

 AND TRACTUS INTERMBDIO-LATERALIS. 



In order to arrive at a clear understanding of the nature of the tract from which 

 the sensitive nerves of the medulla arise, it wiU be necessary to study carefuUy the 

 passage into the medulla of the two remarkable columns of nerve cells found in the 

 posterior cornua of the spinal cord, and described by Clarke under the name of 

 posterior vesicular columns {columnce vesiculosce ptosteriores) and tractus intermedio- 

 lateralis. The posterior or sensitive nerve roots of the cord were shown by Clarke^ 

 and myself to be very intimately connected with the posterior vesicular columns, 

 either directly, or as I have shown to be the case in the lumbar region,^ after pass- 

 ing through the longitudinal columns of the cornua. The tractus intermedio- 

 lateralis which is developed in the dorsal and cervical regions of the cord, seems to 

 be a means of uniting the anterior and posterior cell groups, and serves especially to 

 connect them with the very interesting longitudinal fasciculi, by which the lateral 

 portions of the gray substance are bordered near the junction of the anterior and 

 posterior cornua. 



In his last memoir on the spinal cord (1859), Clarke has traced at considerable 

 length the changes which are observed in the tractus intermedio-lateralis and pos- 

 terior vesicidar columns. Of the latter he states, that in the mammalia, " in the 

 upper part of the cervical region, near the origin of the third pair of nerves, a 

 darker and more defined mass reappears at the base of the cervix cornu. It is 

 composed of cells both large and smaU, and of bundles of the posterior roots which 

 interlace amongst them. This mass is not distinctly circumscribed like that of the 

 posterior vesicular column in the dorsal region, but is somewhat triangular, with 

 one of its angles directed towards the point of the posterior cornu, another towards 

 the transverse commissure, and the third obliquely forwards and outwards towards 

 the antero-lateral column. It gradually diminishes upwards, and disappears near 

 the first pair of nerves." {Philos. Trans., 1859, 447.) 



In the spinal cord of man, as we ascend through the cervical enlargement, " the 

 dark oval masses decrease, and at length disappear; but the spaces which they 

 occupied along the inner halves of the cornua are still interspersed with a multitude 

 of cells, and traversed by the posterior roots. The cells, however, are very much 



' Philos. Transactions, 1859. " Memoirs of the American Academy, 1861. 



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