The Central Nervous System. 185 



2. A small portion of the spinal cord may be excised and examined 

 (preferably under water) for the following: (see p. 22, fig. 18). 



(a) The division of the cord into lateral halves by two median 

 depressions, the anterior median fissure (fissura mediana 

 anterior) and the posterior median sulcus (sulcus medianus 

 posterior) . 



(b) The division of each half into three columns by shallow 

 grooves, the anterior and posterior lateral sulci. The grooves 

 are marked by the attachments of the anterior and posterior 

 nerve roots. The three columns of each half of the cord are 

 the anterior, lateral, and posterior funiculi. 



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

 seen to form a peripheral investment enclosing the grey sub- 

 stance (substantia grisea) of the centre of the cord. The 

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

 being composed of anterior larger and posterior smaller 

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

 as complete structures, the anterior and posterior grey 

 columns. In the median plane, but nearer the dorsal than 

 the ventral surface, is the minute central canal (canalis 

 centralis), the cavity of the spinal cord. 



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

 processes of the frontal bone 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 chief part of the operation consists in removing 

 the temporal portion of the skull by successive steps, exposing first the 

 flocculus of the cerebellum, a small stalked body which is almost com- 

 pletely enclosed by the dorsal portion of the petrosal. The entire petro- 

 tympanic bone is easily detached, and if removed en masse the flocculus 

 and probably also the roots of the facial and acoustic nerves will be 

 destroyed. 



The arches of the first three or four cervical vertebrae should be 

 removed, unless the anterior portion of the cord has been exposed in 

 the previous dissection. 



The spinal cord may be divided at about the level of the third ver- 

 tebra. The brain should then be raised very gently from the ventral 

 wall of the skull and the nerve roots divided with a small sharp knife or 

 with fine scissors. This operation requires considerable care, since the 

 nerves are strongly attached at their points of exit from the cranial wall, 

 but very lightly attached to the brain, so that they are in danger of 

 being torn away. 



The anterior end of the brain may beireed by cutting across the small 

 anterior expansions formed by the olfactory bulbs. 



Portions of the dura mater removed with the brain may be cut away 

 with scissors. It is adherent chiefly along two lines: one representing 

 the longitudinal fissure between the cerebral hemispheres, the other the 



