548 



THE SPINAL CORD AND ITS MEMBRANES. 



The com- 

 missure : 



the grey 

 part, 



with its 

 central 

 canal 



lined by 

 epithelium ; 



the white 

 part. 



The half of 

 cord. 



The grey 

 crescent. 



Posterior 

 cornu : 



its parts. 



Anterior 

 cornu. 



Inter- 



mediate 



process. 



White 

 substance. 



The commissure consists of two parts, viz., a transverse band of 

 grey matter (fig. 198, ft), with a white stratum in front. 



The grey transverse band ( posterior or grey commissure} connects 

 the opposite crescents, and is placed rather nearer the front than 

 the back of the cord. In its centre is the shrunken canal of the 

 spinal cord (fig. 198, i), which is best seen in the foetus. It 

 reaches the whole length of the cord, and a cross section shows it 

 as a round spot. Above, the canal opens on the floor of the fourth 

 ventricle ; and below, it is continued into the filum terminate. 

 It is lined by a columbar ciliated epithelium, and is obstructed 

 by a granular material near the upper end. 



The anterior or white commissure is best marked opposite the 



cervical and lumbar enlargements on 

 the cord, and is least developed in 

 the dorsal region. 



Lateral half. In the half of the 

 cord, as in the commissure, grey and 

 white portions exist ; the former is 

 elongated from before backwards, 

 being crescentic in shape, and is 

 quite surrounded by white matter. 

 The grey matter (fig. 198 </), has 

 its extremities or cornua directed 

 towards the roots of the nerves, 

 and the convexity to the middle 

 line. The crescentic masses in the 

 opposite halves of the cord are 

 united by the grey commissure. 



Taking a cross section of the 

 dorsal region as an example : the 

 posterior cornu is long and slender 

 (fig. 199), and reaches nearly to 

 the surface along the lateral fissure. 



It is rather narrow at its base (cervix, '), and enlarged towards its 

 extremity (caput, '), where it is surmounted by a semi-transparent 

 layer which has been named the substantia gelatinosa (3). There is 

 also on the inner side of the cervix of the posterior cornu a special 

 portion of grey matter containing nerve-cells, the posterior vesicular 

 column of Clarke ( 8 ), which is most developed in the lower dorsal 

 region. 



The anterior cornu (fig. 199) is shorter and thicker than the 

 posterior, and projects towaids the anterior roots without reaching 

 the surface. Its end has an irregular or zigzag outline. 



A third smaller projection of the grey matter is seen in the upper 

 part of the dorsal region of the cord, on the outer side of the crescent, 

 between the anterior and posterior horns : this is known as the 

 intermediate process (Gowers) or the lateral cornu (fig. 199.) 



The white substance of the cord is composed chiefly of medullated 

 nerve-fibres disposed in longitudinal bundles, which are enclosed by 

 irregular septa of connective tissue prolonged from the pia mater on 



FIG. 199. OUTLINE OF THE GREY 

 SUBSTANCE IN THE SPINAL 

 CORD, NEAR THE MlDDLE OP 

 THE DORSAL REGION (LOCK- 

 HART CLARKE). 



1 . Caput cornu posterioris. 



2. Anterior cornu. 



3. Substantia gelatinosa. 



4. Central canal of the cord. 



5. Posterior commissure. 



6. Intermediate process. 



7. Cervix cornu posterioris. 



8. Posterior vesicular column. 



