170 VISION 



distinguished by the most rostral continuation of the cerebellar granular 

 layer dorsomedially, and by nucleus isthmi dorsolaterally (ni, Figure 20A). 

 Nucleus isthmi consists of small spherical and bipolar cells oriented horizon- 

 tally relative to the ventricular sulcus. The connections of nucleus isthmi in 

 chondrichthians are presently unknown. In other vertebrate species, nucleus 

 isthmi receives a topographically organized projection from the optic tectum 

 and projects it back onto the cells of the central tectal zone. The function of 

 this tectal feedback system is related to vision, but its exact significance is 

 unknown. 



At these same caudal tegmental levels, a ventral midline nucleus, the 

 nucleus interpeduncularis (in, Figures 3B, 5B, 20) is recognized. This nucleus 

 receives afferents from the habenular nuclei via the fasciculus retroflexus 

 (fr, Figures 2C, 5A). The efferents of the interpeduncular nucleus are un- 

 determined in cartilaginous fishes, but this nucleus forms extensive descend- 

 ing medullar projections in other vertebrates. 



More rostrally, the medial tegmentum is characterized by a nucleus of 

 large scattered neurons, nucleus interstitialis (in, Figures 3A, 5A). The 

 afferents to this nucleus are unknown, but the axons of its cells form the 

 most rostral component of the medial longitudinal fasciculus, and connec- 

 tions with spinal neurons are known in Dasyatis (R. B. Leonard, personal 

 communication). 



In sharks, the dorsocentral tegmental region is occupied by the intercolli- 

 cular nucleus (ic, Figures 3, 5). In batoids, a distinct intercollicular nucleus 

 was not recognized, but may be represented by the ventral portion of the 

 caudal central tectal zone (cz, Figure 20B, C). The intercollicular nucleus 

 receives spinal input in sharks (Ebbesson 1972). Its efferents are undeter- 

 mined in chondrichthians, but in other vertebrates it projects rostrally into 

 the thalamus, forming an ascending somatosensory pathway. 



The ventrocentral tegmentum consists of two nuclei: a ventrolateral cell 

 group, the lateral tegmental nucleus; and a dorsomedial cell group, nucleus 

 ruber. The lateral tegmental nucleus (It, Figure 20B) consists of small 

 spherical cells in Raja, and spinal input to this nucleus occurs in Scyliorhinus 

 (Hayle 1973a). Its efferents are unknown, but its topographical position 

 suggests that it may be homologous to the substantia nigra of other verte- 

 brates. If so, it will likely receive tectal input and possess reciprocal connec- 

 tions with the dorsal striatum of the telencephalon. 



Nucleus ruber (t, teg, Figures 3B, 20C) consists of medium-sized fusiform 

 and triangular cells. It receives ascending input from the cerebellar nucleus in 

 Ginglymostoma (Ebbesson and Campbell 1973) and gives rise to a crossed 

 descending pathway that reaches spinal levels (R. B. Leonard, personal 

 communication). In other vertebrates, nucleus ruber also receives afferents 

 from the telencephalon; a similar projection may exist in elasmobranchs; 

 Ebbesson (1972) has charted a heavy terminal field in the region of nucleus 

 ruber in Ginglymostoma, following telencephalic lesions. In mammals, 

 telencephalic projections to nucleus ruber arise in primary motor cortex, and 

 the cortico-rubrospinal pathway activates contralateral flexor motor neurons 

 and inhibits contralateral extensor neurons. Thus, the elasmobranch 



