PISCES 175 



the crura cerebri, or cerebral peduncles. They are important 

 conducting tracts of motor and sensory fibres to and from the 

 forebrain. The central canal of the midbrain is called the 

 iter, or aqueduct of Sylvius. 



The iter expands into the fourth ventricle, which is the 

 cavity of the hindbrain. The hindbrain, or rhombencephalon, 

 presents a thin roof except in front, where it is thickened and 

 expanded to form the tongue-shaped cerebellum or meten- 

 cephalon. On each side in front the walls are folded, forming 

 the restiform bodies, and the floor is thickened in continuation 

 with the spinal cord and the crura in front. The thickening 

 constitutes the medulla oblongata and it contains important 

 centres and tracts. The superior tracts are continuous with 

 the sensory tracts of the spinal cord and the inferior ones with 

 the motor tracts of the spinal cord, and these are directed to 

 the cerebellum and to the forward parts of the brain. The 

 centres of the medulla control the heart and the gills. The 

 cerebellum of the fish is large in relation to the great powers 

 of movement. The cerebellum is closely associated with 

 centres of nerves coming from the auditory organ con- 

 cerned with movements relating to equilibrium, and with 

 centres of cutaneous nerves. Movements may be produced 

 through the connexions of the cerebellum with cutaneous 

 nerves and the acustico-lateralis system from stimuli reaching 

 it through the lateral line system, the ear, the eye, the nose, 

 or the mouth. 



The medulla passes insensibly into the spinal cord, and its 

 ventricle narrows to form the central canal of the cord. The 

 walls are divided by dorsal and ventral fissures into lateral 

 halves containing paired centres and tracts. 



The central nervous system shows not merely a differentia- 

 tion into the various parts of the brain, but primitively a 

 segmentation which has been found to be constant. These 

 segments, which alternate with the primitive segments of the 

 upper part of the coelom or myotomes, have been called neuro- 

 meres. 



The nerves arise from neurons which send out branches 

 from the medullary canal, and such are motor nerves. Nerves 

 also arise from the neural crest, which is produced as the neural 



