ELASMOBRANCH BRAIN ORGANIZATION 173 



posterior commissure. These pretectal nuclei correspond topologically to the 

 rostral continuation of the superficial, central, and deep, or periventricular, 

 tectal zones; therefore, they have been named according to these zones. 



At rostral thalamic levels (Figure 2B), a second optic pathway, the medial 

 optic tract, forms by splitting from the marginal optic tract and courses 

 dorsomedially, where it divides into dorsal and ventral fascicles. The dorsal 

 fascicle courses over the intertectal commissure and enters the rostral tectum 

 (Figure 2B). The ventral fascicle continues caudally through the pretectal area, 

 terminating among the cells of both the central and periventricular pretectal 

 nuclei (Figures 2C, D, 4B). Cells of both the periventricular pretectal 

 nucleus and tectum may receive retinal input via the ventral fascicle of the 

 medial optic tract, as both cellular areas possess apical dendrites entering the 

 central pretectal zone. 



All of the thalamic and pretectal nuclei receiving retinal input also receive 

 ascending tectal input (Ebbesson et al. 1972). Most of the remaining dorsal and 

 ventral thalamic nuclei, medial to the retinal and tectal recipient zones, receive 

 ascending cerebellar and spinal inputs (Ebbesson et al. 1972, Ebbesson and 

 Campbell 1973). Thus the thalamus of sharks receives a wide range of ascend- 

 ing sensory pathways, and considerable separation of sensory modalities ap- 

 pears to exist. 



The thalamus of sharks is also known to give rise to sizable ascending 

 pathways that terminate primarily in the central nucleus of the telencephalon 

 (Ebbesson 1972, Schroeder and Ebbesson 1974). The exact origins of these 

 ascending thalamo-telencephalic pathways are unknown; but it is established 

 that thalamic projections are to the contralateral telencephalic hemisphere, 

 rather than to the ipsilateral hemisphere as in other vertebrates. 



At present, nothing is known about the pretectal efferents in chon- 

 drichthians. However, in other vertebrates the pretectum forms complex 

 connections with several different brain regions, such as the tectum, corpus of 

 the cerebellum, tegmentum, and the more rostral dorsal thalamic nuclei, and 

 has been implicated in such diverse functions as eye— head coordination and 

 visual detection of potential predators. 



Little is known about the third division of the diencephalon, the hypo- 

 thalamus. It consists of a rostral preoptic area, a central or tuberal area 

 including the inferior lobes, and a caudal or posterior hypothalamic area 

 (Figures 2-5, 20, 27). Retinal projections are known to the rostroventral 

 preoptic area (Graeber and Ebbesson 1972a, Northcutt 1976); See Figure 

 2B, C. Telencephalic input to the preoptic area and inferior lobes has also 

 been demonstrated (Ebbesson 1972). The lateral lobes form the bulk of the 

 chondrichthian hypothalamus, and considerable variation exists in their 

 organization. 



In chimaeras, sharks, and some batoids (Platyrhinoidis) the inferior lobes are 

 characterized by extensive lateral recesses of the third ventricle, and the lobar 

 nuclei are organized mainly as periventricular laminae (Figures 2, 3, 5, 27C). In 

 the advanced batoids, the lateral recesses are reduced, and distinct nuclear 

 groups replace the periventricular laminae (Figures 20D, 27A). The inferior 

 lobes have been implicated in feeding behaviors (Demski 1977) and in the 



