148 VISION 



floor of the hindbrain (medulla) and midbrain (tegmentum) is much more 

 conservative, and homologous areas are more easily recognized among 

 vertebrates than are areas of the forebrain. The anatomy of each brain region 

 will be summarized, its known connections reviewed, and the variation 

 among chondrichthians noted. 



Brain Stem— The brain stem is formed by the floor of the hindbrain 

 and midbrain, excluding the cerebellum and optic tectum. The sensory and 

 motor columns of the spinal cord continue into the brain stem and terminate 

 at its rostral border, marked by the nucleus interstitialis and the oculomotor 

 nucleus (Figures 5, 20). Thus, the brain stem, like the spinal cord, consists of 

 dorsal sensory nuclei and ventral motor nuclei, as well as pathways passing to 

 and from the spinal cord. 



Recently, Smeets and Nieuwenhuys (1976) described the brain stems of 

 the sharks Squalus and Scyliorhinus and reviewed the earlier literature. A 

 similar treatment of batoids does not exist. Thus, I will deal primarily with 

 the brain stem of Raja, rather than merely repeating details provided by 

 Smeets and Nieuwenhuys. The nomenclature used for Raja is essentially 

 that of Smeets and Nieuwenhuys, and differences between the brain stem of 

 Raja and those of other chondrichthians will be noted. 



The nuclei of the brain stem will be described under the following head- 

 ings: (1) somatic motor nuclei, (2) visceral motor nuclei, (3) reticular forma- 

 tion, (4) visceral sensory nuclei, (5) somatic sensory nuclei, and (6) acousti- 

 colateralis nuclei. 



Somatic motor nuclei— These nuclei comprise a rostral extension of 

 the spinal somatic motor column and consist of a caudal group (Figure 21 A, 

 B) innervating the hypobranchial muscles and a rostral group innervating the 

 extrinsic eye muscles: abducens, trochlear, and oculomotor nuclei. The 

 abducens nucleus (not pictured) consists of a loosely scattered group of 

 medium-size cells extending dorsally from the ventral exit of the abducens 

 nerve roots. This nucleus is located in the brain stem, between the levels 

 indicated in Figure 22B, C. 



The trochlear and oculomotor nuclei are well developed and form a con- 

 tinuous column of cells with no distinct break between the two nuclear 

 groups (Figure 20A, B, C). Raja, like the lampreys, possesses distinct dorsal 

 and ventral divisions of the oculomotor nucleus (Figure 20C). The dorsal 

 division lies just lateral and ventral to the medial longitudinal fascisulus, 

 as in most vertebrates, while the ventral division forms a second distinct 

 nucleus of large fusiform and triangular cells located among the exiting 

 roots of the oculomotor nerve. This peculiar oculomotor pattern has been 

 experimentally confirmed in Dasyatis by Rosiles et al. (1977). 



All of the somatic motor nuclei lie in close proximity to the medial 

 longitudinal fasciculus (mlf, Figures 20-23). The medial longitudinal 

 fasciculus can be traced rostrally to the level of nucleus interstitialis (Figure 

 5) and caudally into the spinal cord. The axons of this pathway both ascend 



