THE NERVOUS SYSTEM 35I 



they acquire sensory centers within the optic lobes of the mesencephalon. 

 Olfactory centers arise in the diencephalon and effect connexion with 

 spinal motor centers to form a mechanism for olfactory reflexes. Also 

 involved in these reflexes are paired habenular ganglia in the roof of 

 the diencephalon, and the interpeduncular nucleus in the base of the 

 mesencephalon. Paired epiphyses project from the roof of the diencepha- 

 lon, and its lateral walls are thickened as the ganglionic masses of the 

 thalami. A funnel-shaped infundibulum projects from the floor towards 

 the roof of the mouth. (Fig. 310) 



The roof of the mesencephalon of cyclostomes is peculiar in having a 

 chorioid plexus which serves as a means of nourishing the brain. Lateral 

 to this plexus are the conspicuous paired swellings of the optic lobes, 

 the centers of vision. The thickened lateral wall of the mid-brain is 

 largely fibrous and is known as the tegmentum. The floor of the mid- 

 brain contains the motor center of the oculomotor nerve, which innervates 

 four of the eye muscles. The functions of the mid-brain are predominantly 

 locomotor and somatic rather than visceral. 



From the roof of the metencephalon arises an inconspicuous cerebel- 

 lum, which is the anterior continuation of the lateral-line centers of the 

 myelencephalon. In it are located the complex Purkinje cells, which 

 have fiber connexions with motor cells in the medulla and cord, and are 

 found in the cerebellum of all vertebrates. The lateral walls and base 

 of the metencephalon consist largely of fiber tracts, most of which are 

 ascending and descending fibers which connect brain and spinal cord. 

 No pons, which is so prominent a feature of the base of the mammalian 

 metencephalon, is present in cyclostomes or in any of the lower vertebrates. 



The myelencephalon or medulla oblongata is a transitional region 

 between brain and spinal cord. Its roof is largely differentiated as a 

 chorioid plexus. The lateral walls contain the sensory centers of the 

 lateral-line nerves as well as those of other cranial nerves. The motor 

 centers of the trigeminal, facial, glossopharyngeal, and vagus nerves are 

 located in the medulla. 



The lumen of the brain expands into four large ventricles, two of which 

 lie in the paired divisions of the telencephalon, the third is in the dienceph- 

 alon, and the last in the myelencephalon. Three fiber tracts or com- 

 missures connect the right and left halves of the brain, the anterior 

 in the wall of the telencephalon, the habenular in the roof of the dien- 

 cephalon and the posterior in the roof of the mesencephalon. All three 

 persist throughout the vertebrate series to man, and serve as important 

 landmarks by which to determine homologies. 



In addition to the numerous nervous structures which emerge in 

 the cyclostome brain, the hypophysis acquires intimate relations with 

 the infundibulum, and shows the first stages in the formation of the 



