EMBRYOLOGY AND COMPARATIVE ANATOMY OF BRAIN. 29 



characterized the cerebellum in all classes above the selacians. 

 Directly caudad of the cerebellum in the roof of the hind-brain 

 we meet with ganglionic masses, which give off fibres to the 

 trigeminal and the acoustic nerves. Fused closely with the 

 cerebellum they are only small nuclei in mammals, but in fishes 

 they form quite respectable lobes. 



From the thalamic region to the end of the spinal cord the 

 central cavity (ventricle, central canal) is surrounded with masses 

 of gray substance, rich in ganglion-cells, and caudad of the 

 mid-brain we find in this gray matter the peripheral nerves 

 arising from their nuclei. In the gray substance of the floor 

 of the hind- and after- brain arise a great portion of the fibres 

 of the trigeminal and abducens, a part of the acoustic and of 

 the vago-glosso-pharyngeal. Somewhat nearer the surface lies 

 a column of nuclear cells, from the upper part of which the 

 motor root of the fifth pair arises, and farther caudad the facial 

 nerve. From the hind-brain to the sacral portion of the spinal 

 cord there is an unbroken series of nuclei. There are reallv 



/ 



two series, one more ventral (zone of anterior horn, His) and the 

 other more lateral (lateral zone). From the former arise the 

 hypoglossal and all the anterior roots of the spinal nerves going 

 to the muscles of the trunk. From the latter there arise 

 (Gaskell) fibres which are concerned in the innervation of the 

 muscular coat of the viscera. These lateral fibres leave the 

 central organ along with the anterior-horn fibres, except in the 

 region of the medulla, where they go to form the motor vagus 

 and the accessory nerves. Farther down they leave the spinal 

 cord in company with the other fibres of the anterior roots 

 According to Gaskell the latter pass into the mixed nerves 

 while the former are connected with the sympathetic. 



In the ventral portion of the pons and oblongata there are 

 situated many aggregations of ganglion-cells and nerve-fibres, 

 whose relations, varying widely throughout the animal kingdom, 

 cannot be discussed here. It is of importance that all the nerve- 

 fibres which connect the brain with the centres situated below 



