ELASMOBRANCH BRAIN ORGANIZATION 155 



Two additional groups of cells, the inferior olive (io, Figure 21A, B, C) 

 and the nucleus of the lateral funiculus (ne, Figure 21B), are closely as- 

 sociated topographically with the reticular formation and are probably 

 phylogenetically derived from this formation. The inferior olivary nucleus 

 consists of small, spherical cells embedded in a densely staining neuropil. 

 The efferents of this cell group in chondrichthians are unknown, but a 

 similar nucleus in bony fishes projects to the corpus of the cerebellum (un- 

 published observations) and is probably homologous to the accessory olivary 

 nuclei of mammals. 



The nucleus of the lateral funiculus consists of small- to medium-sized 

 fusiform or bipolar neurons scattered among the fibers at the outer edge of 

 the ascending lateral or spinal lemniscus. The efferents of this nucleus are 

 unknown in cartilaginous fishes, but in mammals a similarly situated nucleus 

 projects to the cerebellar vermis. 



Visceral sensory nuclei— The visceral sensory fibers of VII, IX, and 

 X— carrying gustatory, pain, temperature, pressure, and respiratory chemo- 

 receptor information— terminate in a longitudinal column of cells dorso- 

 lateral to the visceral motor column, termed the vagal lobe (vl, Figures 

 21, 22B). The lobe consists of small- to medium-sized, spherical or fusiform 

 neurons scattered among fascicles of the visceral sensory fibers. Rostrally the 

 vagal lobe gives rise to a well-encapsulated bundle, the secondary gustatory 

 tract (gt, Figure 22C), which ascends rostrally to a position medial to the 

 nucleus intermedius of the posterior lateral line lobe, where it can no longer 

 be recognized as a distinct bundle. No details about the higher order organi- 

 zation of the visceral sensory systems in chondrichthians are available, but in 

 bony fishes the vagal lobe projects, via the secondary gustatory tract, to a 

 distinct secondary gustatory nucleus located ventral to the cerebellar 

 corpus and nuclei. Medial to the cerebellar nucleus, a distinct nucleus z 

 (Figure 23D) may represent a secondary gustatory nucleus in Raja, but 

 experimental study is needed to test this possibility. In other fishes, the 

 secondary gustatory nucleus projects to the caudal inferior lobe of the 

 hypothalamus (Finger 1976). Whether or not gustatory information reaches 

 telencephalic levels in fishes is unknown. 



Somatic sensory nuclei— The dorsal column nuclei, the nucleus of 

 the descending tract of V, the principal trigeminal nucleus, and the 

 trigeminal mesencephalic nucleus constitute the somatic sensory systems of 

 the brain stem. In land vertebrates, the dorsal column nuclei (nuclei cuneatus 

 and gracilis of mammals) are located in the dorsal obex region of the medulla. 

 They receive afferents via the dorsal funiculus of the spinal cord, and their 

 axons give rise to the medial lemniscus after crossing the medullar midline as 

 the internal arcuate fibers. According to Hayle (1973a), the dorsal funiculus 

 of Scyliorhinus does not possess long ascending fibers, nor were Smeets and 

 Nieuwenhuys (1976) able to recognize distinct dorsal column nuclei in 

 Scyliorhinus or Squalus. However, Ebbesson (1972) traced dorsal funicular 

 fibers to an obex cell group he termed the dorsal column nuclei, following 



