164 VISION 



body parts. Lesions of the auricle result in severe locomotor impairment in 

 Squalus (Karamyan 1962). Animals with auricular lesions roll on the long 

 axis and/or swim in circles, turning toward the side of the lesion. However, 

 animals with bilateral auricular lesions swim normally. 



Similar lesions in Raja clauata do not produce the same effects (Karamyan 

 1962). After removal of the corpus and cerebellar nucleus, no marked motor 

 disorders are noted. Posture of the skates is normal, although some irregular 

 contractions of fin segments are noted, and motor activity increases. Lesions 

 involving the auricle produce marked postural changes, particularly in 

 moving skates. These animals swim with rostrally elevated bodies, often 

 swimming almost vertically. 



Internal organization of the chondrichthian cerebellar cortex (Figure 20B) 

 is similar to that of other vertebrates. Elasmobranchs possess all the cell 

 types found in mammals, except for basket cells and a well-developed 

 inhibitory plexus formed by Purkinje axon collaterals (Nicholson et al. 

 1969). Details of cerebellar neuronal anatomy and physiology have been 

 recently summarized and reviewed by Nicholson et al. (1969), Paul (1969), 

 and Tsukahara (1969). 



Ebbesson and Campbell (1973) examined the cerebellar efferents in 

 Ginglymostoma and demonstrated both ascending and descending cerebellar 

 pathways like those found in other vertebrates. The Purkinje cells of the 

 cerebellum of sharks, like those of other vertebrates, terminate on deep 

 cerebellar nuclei, but direct projections to the vestibular nuclei or medulla, 

 suggested by earlier studies (Kappers et al. 1936), were not confirmed. 

 The cerebellar nuclei terminate in the lateral and medial medullar reticular 

 formations, the red nucleus of the tegmentum, the trochlear and oculomotor 

 nuclei, and the posterior dorsal thalamus. Thus, the cerebellum possesses 

 descending projections, modulating the output of the reticular formation to 

 the cranial nerve motor nuclei and to the spinal cord, and an ascending 

 brachium conjunctivum that reaches dorsal thalamic levels. This last projec- 

 tion is particularly interesting, as it suggests that thalamo-telencephalic relay 

 of cerebellar afference may exist. In other vertebrates the thalamo- 

 telencephalic projection is to a motor area of isocortex. Demonstration 

 of such a pathway would further strengthen the probability of telencephalic 

 involvement in direct motor control suggested by Ebbesson 's demonstration 

 of direct telencephalic pathways to medullar nuclei (Ebbesson 1972). 



Mesencephalon— The midbrain of chondrichthians consists of dorsal 

 (optic tectum and torus semicircularis) and ventral (tegmentum) regions 

 (Figures 2-5, 20, 27). The optic tectum consists of multiple laminae, 

 frequently grouped into zones, but no agreement exists regarding the exact 

 number (Table 5). I have used the nomenclature of Gerlach (1947) in this 

 chapter and have grouped his layers into three zones: periventricular, central, 

 and superficial (Figures 2-5, 20, 27). 



The periventricular tectal zone (Figures 3, 4, 20, 27) consists of an un- 

 myelinated fiber layer (layer 1) and two or more cellular laminae (layer 2). 

 Pyriform cells predominate in this zone, and their apical dendrites branch 



