ELASMOBRANCH BRAIN ORGANIZATION 157 



populations of mesencephalic cells: a large group, functioning as a sensory 

 neuron in a monosynaptic jaw adduction reflex; and a smaller group, an 

 interneuron-cerebellar pathway of unknown function. 



Acousticolateralis nuclei— The acousticolateralis nuclei and their 

 associated nerves form a prominent complex in the rostral hindbrain. This 

 complex is frequently termed the acousticolateralis area, or, in the older 

 literature, the acousticolateralis tubercle (Kappers et al. 1936). An under- 

 standing of the complex relationships of this area requires examination of 

 the cranial nerves associated with the acousticolateralis area. Three cranial 

 nerves terminate in the various nuclei forming the acousticolateralis area: 

 anterior and posterior lateral line nerves and a statoacoustic nerve (Figures 8, 

 9, 12, 13, 16, 17). The anterior and posterior lateral-line nerves innervate the 

 lateral line and associated sensory organs of the head and trunk, respectively. 



The lateralis nerves are not numbered because until recently they were 

 believed to be components of the facial and vagal nerves (Kappers et al. 

 1936). However, embryological evidence has demonstrated that the lateralis 

 nerves arise from a dorsolateral series of head placodes, as does the stato- 

 acoustic nerve, and that together these nerves form a series that parallels the 

 branchiomeric nerves, rather than comprising components of the branch- 

 iomeric series. This new view is strongly reinforced by the facts that the 

 lateralis nerves possess separate and distinct ganglia (of different embryonic 

 origin than those of the branchiomeric nerves) and that they enter the 

 medulla separately and terminate in sensory nuclei distinct from any of the 

 branchiomeric nerves (McCready and Boord 1976, Boord and Campbell 

 1977). 



The anterior lateral line nerve innervates the mechanoreceptors (neuro- 

 masts) of the ordinary lateral line, as well as the electroreceptors located 

 on the head and termed ampullae of Lorenzini. The peripheral distribution 

 and central roots of the anterior lateral line are summarized in Figures 12, 

 24, 25. The anterior lateral-line nerve consists of four peripheral branches: 

 superficial ophthalmic, buccal, otic, and external mandibular. These rami 

 are closely associated peripherally with rami of cranial nerves V, VII, and 

 VIII but do not anastomose with these rami, and each lateralis ramus 

 possesses its own distinct ganglion (McCready and Boord 1976). As each 

 peripheral lateralis branch or ramus approaches the medulla it divides into 

 dorsal and ventral roots. The dorsal root carries only ampullary fibers, and 

 the ventral root only mechanoreceptor or neuromast fibers. The dorsal root 

 enters the anterior lateral-line lobe (Figures 16, 25, 26), and the ventral 

 root enters the posterior lateral-line lobe (Boord and Campbell 1977). 



The posterior lateral-line nerve innervates the trunk neuromasts and con- 

 sists of a single root as it approaches the medulla and enters only the 

 posterior lateral-line lobe. Thus the anterior lateral-line lobe receives electro- 

 receptive information only via the ampullary system, and the posterior 

 lateral line lobe receives mechanoreceptive information from the head and 

 trunk neuromasts (Boord and Campbell 1977). 



