MECHANORECEPTORS AND BEHAVIOR 359 



The area of the body supplied by one segmental spinal nerve— the sensory 

 dermatome— has been determined for Scyliorhinus by Rijnberk (1904) and 

 Roberts (19696) (Figure 16) and for rays by ten Kate (1928). Unlike the 

 motor nerves, which supply only adjacent motor segments (Bone 1964), the 

 sensory fibres distribute over a number of adjacent segments, so that each 

 dermatome overlaps with its neighbour. 



Figure 16 The form of the dermatome of one abdominal segment (arrow) of Scylio- 

 rhinus. Note the trapesoidal shape and the extent of the dermatome. (Roberts 1969ft). 



Acoustico-Lateralis System 



Sensory Centres of the Lateral-Line— Much of the earlier work on the anat- 

 omy of the cranial nerves of elasmobranchs was carried out in the cause 

 of developing an evolutionary theory of the vertebrate head. Therefore, it 

 does not deal specifically with lateral-line innervation, which is complicated 

 by the parallel distribution of nerves V and VII and, in elasmobranchs, by 

 the presence of the ampullae of Lorenzini. 



An important step was taken by Marshall and Spencer (1881), who 

 demonstrated that the buccal nerve, which supplies the infraorbital canal, 

 was part of the facial nerve (VII) and was not a component of nerve V. This 

 led to the conclusion that the trigeminal nerve (V) did not supply the lat- 

 eralis system, and to Ewart's classification of the canals, based on their 

 innervation, which we have already considered. Cole (1896) discusses some 

 possible exceptions to this pattern of innervation. 



An excellent description of the innervation of the head canals is provided 

 by Norris and Hughes (1920) for Squalus, but the pattern is common to 

 most elasmobranchs. The lateral-line organs of the head canals (and the 

 ampullae of Lorenzini) are supplied by fibres of the anterior lateral-line 

 nerves (superficial ophthalmic, buccal and external mandibular branches) 

 which are intermingled in nerve VII with the fibres serving other sensory 

 modalities, and the lateral canal is innervated by the posterior lateral-line 

 nerve, which enters the medulla in association with nerve X. The diagram of 

 Figure 17 shows the arrangement in Squalus, where some of the lateral canal 

 sense organs are supplied by nerve IX as well. There are about 6000 lateral- 

 line fibres feeding into the lateral-line lobes in Scyliorhinus, mostly about 12 

 [Jtm diameter; the cell bodies of these fibres cluster in ganglia peripheral to 

 the brain. 



