THE SPINAL CORD 347 



lateral or anterior and posterior lateral sulci, of which 

 only the latter are at all distinct. The grooves are marked 

 by the attachments of the ventral and dorsal nerve roots. 

 The three columns of each half of the cord are the ventral 

 (anterior), the lateral, and the dorsal (posterior) funiculi. 



(c) On the cut surface the white substance (substantia alba) 

 is seen to form a peripheral investment enclosing the grey 

 substance (substantia grisea) of the centre of the cord. 

 The grey portion is somewhat H-shaped in section, each 

 half being composed of ventral larger and dorsal smaller 

 masses, known in section as the horns of the grey matter, 

 or, as complete structures, the ventral and dorsal grey 

 columns. These grey columns are situated opposite the 

 ventrolateral and dorsolateral sulci of the surface and 

 separate internally the three funiculi mentioned in the 

 previous paragraph. The white substance is composed of 

 ascending and descending nerve fibres and, when a large 

 proportion of these connect the brain with the various levels 

 of the spinal cord, their number necessarily increases in a 

 caudocephalic direction. In the rabbit, however, the propor- 

 tion of such fibres is not great enough for the caudocephalic 

 increase of the white matter to be marked (Fig. 41, p. 77). 

 In the median plane is the minute central canal (canalis 

 centralis), the cavity of the spinal cord. 



(d) The ventral (anterior) spinal artery runs along the cord in 

 the ventral median fissure, giving off branches into the 

 fissure and smaller branches over the surface. 



3. The brain may be exposed by breaking away the supraorbital 

 processes of the frontal bones and then removing the roof of the 

 skull with bone forceps. In order to clear the brain and the 

 roots of the cerebral nerves, it is necessary to remove the entire 

 lateral wall of the skull on both sides. The part of the operation 

 requiring most care is the removal of the temporal portion of 

 the skull by successive steps, exposing first the paraflocculus of 

 the cerebellum (Fig. 42, p. 79), a small stalked body which is 

 almost completely enclosed by the dorsal portion of the petro- 

 sal bone. The entire petrotympanic bone is easily detached, and 



