156 VISION 



hemicordotomy at the second spinal segment in the nurse shark, Ginglymos- 

 toma. 



Distinct dorsal column nuclei were not identified in Raja, and further 

 experimental studies are needed to assess the development of the medial 

 lemniscal system in cartilaginous fishes. 



The entering sensory fibers of the trigeminal nerve form ascending and 

 descending tracts in Raja. The ascending fibers terminate in the principal 

 sensory trigeminal nucleus (pv, Figure 23B, C, D) located in the rostral 

 medulla. The principal nucleus consists of medium-sized polygonal cells 

 embedded in a dense neuropil. Smeets and Nieuwenhuys (1976) were 

 unable to recognize a principal nucleus in Scyliorhinus, and tentatively 

 identified such a nucleus in Squalus as nucleus C. 



In mammals the principal nucleus projects to the thalamus, which in turn 

 projects to primary sensory cortex. In birds and reptiles the principal nucleus 

 projects directly to a rostral basal nucleus of the telencephalon, which in 

 turn projects to the more dorsally located pallium. No specific experimental 

 data exist for the central projections of the trigeminal nerve in cartilaginous 

 fishes. However, Schroeder and Ebbesson (1974) observed a bilateral ascend- 

 ing pathway to a medial subpallial area immediately dorsal to area super- 

 ficialis basalis following mesencephalic lesions in nurse sharks. This pathway 

 may represent a trigemino-telecephalic pathway as in birds and reptiles. 



The descending fibers of the trigeminal nerve form a distinct descending 

 trigeminal tract (dt, Figures 21-23), which is traced caudally to the obex 

 region of the medulla. Medium-sized neurons are scattered in the dorso- 

 medial angle of the descending tract, throughout its rostrocaudal extent, and 

 constitute a nucleus of the descending trigeminal tract (dv, Figures 21, 22, 

 23A). In other vertebrates, somatic sensory fibers of the Vllth, IXth and Xth 

 cranial nerves also contribute to the descending trigeminal tract and nucleus, 

 and thus constitute a major system relaying somatic pain and temperature in- 

 formation to the forebrain. Again, comparable information about chondri- 

 chthians is wanting. 



The trigeminal mesencephalic nucleus consists of large primary sensory 

 neurons whose cell bodies are in the periventricular zone of the optic tectum 

 (mV, Figure 20C). Witkovsky and Roberts (1975) analyzed the mesencephalic 

 trigeminal nucleus in Scyliorhinus, Mustelus, and Raja, concluding that the 

 nucleus consists of at least two populations of cells. The peripheral neurite of 

 a rostral population was observed entering the trigeminal nerve innervating 

 the teeth and adjacent skin of the jaws. These cells are also said to possess a 

 collateral process ending on the neurons of the trigeminal motor nucleus and 

 forming a monosynaptic reflex mediating jaw closure (Roberts and 

 Witkovsky 1975). A second, more caudal population (approximately 15% of 

 the mesencephalic "trigeminal" neurons) was not observed to send a neurite 

 into the trigeminal nerve but was seen to project caudally and medially to 

 terminate among the Purkinje cells of the anterior lobe of the cerebellum. 

 Roberts' and Witkovsky's studies suggest the existence of two distinct 



