154 VISION 



they did not recognize a distinct inferior raphe nucleus. No experimental 

 data exist for the raphe nuclei of chondrichthians, but these nuclei in other 

 vertebrates are characterized by marked 5-hydroxytryptamine (5-HT) 

 activity, and they form extensive connections with both spinal cord and 

 forebrain nuclei. 



The medial reticular zone consists of both medium and large polygonal 

 neurons and occupies a ventrolateral position beneath the medial longitudinal 

 fasciculus (Figures 21-23). The smaller cells form a more or less continuous 

 column along the entire length of the hindbrain, but the large neurons are 

 discontinuous and can be divided into inferior, medial, and superior large- 

 celled components. These divisions were first recognized by van Hoevell 

 (1911) in Raja and have been since confirmed by Smeets and Nieuwenhuys 

 (1976) in Squalus and Scyliorhinus. The inferior large-celled reticular 

 nucleus (inf. Figures 21C, 22B) occupies the caudal medulla, adjacent to the 

 caudal visceral motor column, and is replaced more rostrally by the medial 

 large-celled reticular nucleus (mrf, Figures 22D, 23C). The medial nucleus 

 occupies the medullar region marked by the entry of the statoacoustic and 

 trigeminal nerves, and is replaced rostrally by the superior large-celled 

 reticular nucleus (srf, Figure 23D), which extends through the rostral 

 medulla into the caudal midbrain isthmic region. 



The lateral reticular zone consists of small neurons located between the 

 large-celled medial reticular zone and the descending tract of V (lrz, Figures 

 21-23), and it is likely homologous to the parvicellular reticular nucleus of 

 mammals. The lateral reticular zone is the terminal site for most of the 

 ascending spinal pathways forming the spinal lemniscus (Hayle 1973a) and is 

 also the target of an ipsilateral descending cerebello-bulbar pathway 

 (Ebbesson and Campbell 1973). The neurons of the large-celled medial 

 reticular zone have long, branching dendrites that extend into the lateral 

 reticular zone. Thus, in all likelihood, they also contact fibers of the ascend- 

 ing spinal lemniscus. 



No experimental studies exist for the reticular formation of chondri- 

 chthians, but in other vertebrates the reticular formation is known to form 

 extensive ascending and descending connections with the spinal cord and 

 higher neural centers. The inferior medullar reticular region consists of 

 neuronal populations that give rise to a ventral noradrenergic pathway 

 projecting to the midbrain roof, thalamus, hypothalamus, and basal regions 

 of the telencephalon. In addition, the large-celled reticular zone is the pri- 

 mary target of telencephalic, tectal, and cerebellar efferents linking the 

 brain to the spinal cord by descending reticular pathways. This pattern of 

 connectivity places the reticular formation in an ideal position to function 

 as a pattern generator. The reticular formation consists of both ipsilateral 

 and contralateral pathways projecting to most parts of the spinal cord. 

 Such reticular neurons, projecting over wide segments of the cord and motor 

 nuclei of the medulla, could produce a wide range of output signals depend- 

 ing on the temporal and spatial summation of arriving afferents from very 

 diverse centers of the spinal cord, cerebellum, tectum, hypothalamus, and 

 telencephalon. 



