144 MR HAROLD AXEL HAIG ON THE 



postero-median fissure, which normally is of greater extent than the anterior, would 

 thus be reduced in length. 



The. white matter of the cord appears to consist of very fine medullated nerve-fibres : 

 in the posterior region, the columns of Goll (mesial) and Burdach (lateral) are very 

 well marked, the pia mater dipping into the cord and marking off these two wedges of 

 white matter. The pial septum, separating the postero-lateral column from the crossed 

 pyramidal and direct cerebellar tracts, passes almost to the postero-lateral horn of 

 grey matter. 



The fibres of the anterior roots of the spinal nerves pass out through the white 

 matter in three or four main bundles : those of the posterior roots in one thick bundle 

 of fibres situated at the middle of the postero-lateral horn. 



Comparison with higher type of mammalian cord (cervical region}. 



The main points in the comparative histology are the following : 



(a) The early appearance of the nucleus gracilis, which in the human cord does 



not appear before the medulla has -been reached. 



(b) The extensive spreading out of the grey matter of the posterior and intermedio- 



lateral horns. 



(c) The wide disproportion between the transverse and the antero- lateral diameter 



of the cord, due account being of course taken of any pressure which may 

 have arisen. 



(d) The marked development of the formatio reticularis. 



(e) The relatively enormous size of the motor cells in the anterior cornu : this is 



perhaps not sufficiently emphasised in text-fig. 1, but is nevertheless a very 

 striking feature in the actual section. This feature is possibly related to 

 the highly developed powers of locomotion shown by members of the 

 seal family. 



The proportion of grey to white matter is as 3 to 4 approx. 



B. Dorsal region. (Text-fig. 2.) 



The cord appears in this region to have been submitted to mechanical pressure 

 in the transverse diameter : for this reason, the central canal appears elongated 

 antero- posteriorly and somewhat distorted. The postero-median fissure is longer 

 than the anterior and reaches to the grey commissure ; the anterior fissure stops short 

 of the central grey matter, leaving a well-defined anterior or white commissure. The 

 antero-posterior diameter appears greater than the transverse. The grey matter is 

 divided in each lateral half into three main masses, viz. : 



(a) A posterior horn, in which are to be seen a few medium-sized nerve-cells. 

 (6) An intermedio-lateral horn, at the tip of which a group of medium-sized nerve- 



(ROY. soc. EDIN. TRANS., VOL. XLVIII., 852.) 



