THE NERVOUS SYSTEM 



511 



eflferent fibers which pass as a bundle from the tegmentum or lateral wall 

 of the midbrain to the floor of the medulla and cross to the other side by 

 way of the ansulate cormnissure. In the tegmentum are located also 

 a number of giant cells, the large neurites of which extend along the spinal 

 cord to tail muscles and thus form a physiological through-way. In the 

 floor of the midbrain lies the oculomotor nucleus and the interpeduncular 

 ganglion. In cyclostomes, as in higher vertebrates, the floor of the mid- 

 brain serves as a pathway for fiber tracts leading to and from the hemi- 

 spheres. One of the ascending fiber tracts is the spinothalamicus. 



The thickening of the tegmentum in amphibia constricts the lumen 

 to a slender tube, the aqueduct, which connects third and fourth ventricles. 



In reptiles some of the optic nerve fibers, instead of connecting with 

 the optic lobes as in lower vertebrates, become connected, indirectly by 

 way of synapses within the thalami, with the posterior part of the cerebral 

 hemispheres. This shift in brain connexions begins in reptiles and 

 reaches its climax in mammals. (Fig. 484) A new nucleus, the nucleus 

 ruber, so-called because of red granules associated with it, first appears in 

 reptiles. It is a way station or association center for fibers which carry 

 impulses from the cerebellum to the cerebral cortex. In snakes the optic 

 lobes or corpora bigemina become subdivided into corpora quadrigemina. 



The midbrain of mammals differs little from that of reptiles. Corpora 

 quadrigemina are present in all mammals. The sensory centers of vision 

 shift to the occipital lobes of the hemispheres. The upper swellings or 

 superior colliculi of the corpora quadrigemina are optic reflex centers. 

 The inferior colliculi are auditory reflex centers. They correspond to the 

 tori of reptiles, which are covered by the optic lobes and are thought to 

 be derived from the roof of the brain posterior to the optic lobes of fishes. 



Metencephalon. The metencephalon is hardly distinguishable in 

 cyclostomes, but is represented by a rudimentary cerebellum and a 

 transitional region connecting mesencephalon with the widely expanded 

 medulla. The cerebellum appears to be the anterior continuation of the 

 equilibratory centers of the medulla. Since the cerebellum receives 

 fibers from the medulla, thalami, and optic lobes, it may correlate olfac- 

 tory, visual, and static functions. A decussation of fibers in its floor 

 may possibly be interpreted as the beginnings of a pons. The floor, 

 however, serves chiefly as a pathway for ascending and descending fiber 

 tracts which connect brain and spinal cord. 



In fishes there is great variation in the size of the cerebellum. In the 

 dipnoi it is small and covered by the optic lobes. In some elasmobranchs 

 it becomes conspicuous and even convoluted. In teleosts it invaginates 

 into the cavity of the midbrain to form a valvula. The peculiar Purkinje 

 cells, which are such conspicuous elements in the cortex of the cerebellum 

 of higher vertebrates, occur in fishes. The numerous sensory and motor 



