ELASMOBRANCH BRAIN ORGANIZATION 153 



Visceral motor nuclei— The motor nuclei of X, IX, VII, and V with 

 the Edinger-Westphal nucleus form a ventrolateral column of motor 

 neurons (Figures 20-23) throughout the brain stem. The motor nuclei of 

 X, IX, and VII consist of a single continuous column of large neurons in the 

 caudal medulla (Figures 21, 22B), while the motor nucleus of V (Figure 

 23A, B, C) is located more rostrally as a separate group. 



Rosiles et al. (1977) confirmed experimentally that the motor nuclei of 

 X, IX, and VII do arise from a single caudal brain stem group in Dasyatis. 

 There is some variation in the organization of the caudal visceral motor 

 nuclei in chondrichthians. In chimaeras, the vagal portion of the motor 

 column can be divided into separate rostral and caudal divisions (Addens 

 1933). Addens suggested that the rostral division might be branchiomotor 

 and the caudal division splanchnic motor— homologous in land vertebrates 

 to the nucleus ambiguus and the dorsal motor nucleus of the vagus, 

 respectively. 



In Squalus, Smeets and Nieuwenhuys recognized a lateral motor nucleus 

 of the vagus, in addition to the usual more medial motor nucleus recognized 

 in fishes. A similar condition may exist in Raja, as a distinct group of large 

 neurons is seen among the roots of the vagus (Figure 21) and is termed 

 nucleus w. These cells in Raja may constitute a lateral motor nucleus of the 

 vagus, or primary sensory neurons similar to the Rohon-Beard cells of the 

 spinal cord and mesencephalic trigeminal nucleus. A similar cell group is 

 lateral to the motor nucleus of V in Raja and is identified as nucleus y 

 (Figure 23 A, B). The cells of this nucleus might be caudally situated neurons 

 of the mesencephalic nucleus of V, but they do not look like the more 

 rostrally located mesencephalic V neurons. Nucleus y might also be a lateral 

 division of the motor nucleus of V, or, possibly, a homologue of the locus 

 coeruleus of land vertebrates. 



In Squalus, Smeets and Nieuwenhuys (1976) also recognized an additional 

 nucleus, rostral to the motor nucleus of VII, that may be visceral motor in 

 nature (their nucleus A). They did not recognize a similar group in 

 Scyliorhinus, nor have I been able to recognize such a nucleus in Raja. 



The most rostral nucleus of the visceral motor column is the midbrain 

 nucleus of Edinger-Westphal, situated dorsolateral to the dorsal division of 

 the oculomotor nucleus (Figure 20C). This nucleus consists of small neurons 

 and probably gives rise to the autonomic preganglionic axons innervating 

 the dilatator iridis muscle of the eye. 



Reticular formation— The reticular formation extends the length of 

 the hindbrain and consists of three longitudinal zones: a median or midline 

 zone, a medial large-celled reticular zone, and a lateral small-celled reticular 

 zone. 



In Raja, the median or midline reticular zone consists of inferior and 

 superior raphe nuclei (Figures 21B, 22B, 23B, C, D) composed of medium to 

 large neurons whose cell bodies straddle the midline with dendrites extend- 

 ing into both halves of the medulla. A superior raphe nucleus was identified 

 in Squalus and Scyliorhinus by Smeets and Nieuwenhuys (1976). However, 



