422 Central Nervous System in Vertebrated Animals. [Mar. 8, 



occur at other places as in the fibrse rectse, and in the field of the 

 ventral columns. 



The transverse commissures are — one in the spinal cord which 

 passes through the substantia gelatinosa centralis over the central 

 canal ; another occurs on the ventral side of the anterior part of the 

 medulla oblongata and corresponds to the commissura ansulata of 

 Teleostei ; it is connected with the commissure in the dorsal part of 

 the optic lobes. Two other commissures are present corresponding 

 respectively to the anterior and posterior commissures of the third 

 ventricle of Mammalia. 



There is no ehiasma of the optic nerve visible externally; what 

 there is of it is situated in the substance of the thalamencephalon. 



The anterior root of the fifth nerve arises from a ganglion occupy- 

 ing a broad swelling in the lateral part of the grey matter of the floor 

 of the fourth ventricle.. The posterior root arises from the summit 

 of the restiform bodies. 



The facial passes backwards in a small tubercle at the junction of 

 the floor of the fourth ventricle with the restiform bodies. 



The acusticus arises from a bundle of fibres which are situated on 

 the summits of the ventral columns, and appears to be a continuation 

 forward of that part of the multiaxial fibre which has not decussated. 



The vagus has five roots ; they pass backward and enter in succes- 

 sion the same tubercle as,, and to the outside of, the facial nerve ; the 

 three posterior roots are double, so that the vagus is equivalent to 

 eight nerves, and consists entirely of dorsal roots. 



Two nerves are given off from the ventral side of the medulla 

 oblongata, each of which has two roots ; they do not join the vagus 

 but pass back some distance in the vertebral canal and emerge on a 

 level with the exit of the dorsal roots of the spinal nerves. 



The second and third spinal nerves supply the pectoral fin and 

 pursue the course usually followed by the hypoglossal w T hen that nerve 

 is present in Teleostei, 



The fibres of the ventral roots of the spinal nerves enter in a direc- 

 tion upward and forward toward the inner edgj of the multiaxial 

 fibre, between it and the central canal, and then passing over the dorsal 

 edge of the same, are either lost in the grey substance of the ventral 

 horn, join a process of one of the multipolar cells, or become one of 

 the longitudinal fibres of the ventral column. 



The brain of Cemtodus presents an embryonic condition in three 

 respects, viz., first in the extreme size of the ventricles and in the 

 tenuity of the substance of their walls; second, in the alternating 

 origins of the dorsal and ventral roots ; third, in the fact that the 

 origins of the dorsal roots are close to the central line. 



Compared to Protopterus it differs in the shape and the imperfec- 

 tion of the cerebral lobes, and in the fact of its having a well-deve- 



