172 ORGANIZATION OF THE HEAD v. 16- 



of units. These results suggest that the ear may function as a vibration 

 receptor, but there are no conditioning experiments to show whether 

 these fishes can hear. 



There is a well-developed system of lateral line organs, whose 

 function is considered later (p. 218). The organs of this system on the 

 head are highly modified in elasmobranchs to form the ampullae of 

 Lorenzini, long canals filled with mucus. Sand showed that these 



Fig. 109. Drawing of a sympathetic ganglion and related structures in a dogfish. 

 Lettering as in Figs. 104 and 105. s.o. sense-organs. (After Young, Quart. J. Alter. Sci. 75.) 



organs increase their discharge of nerve impulses with very slight 

 falls of temperature, and he suggested that their function is to detect 

 such changes. They are also sensitive to weak tactile stimulation and to 

 small voltage gradients in the water. Their function therefore remains 

 uncertain. It may be related to determining changes of hydrodynamic 

 pressure distribution over the surface of the aerofoil-like body, 

 especially in skates and rays. They may thus act as mechano-receptors 

 detecting local changes of pressure near the body surface. 



No doubt elasmobranchs, like other animals, have many senses 

 referred to the skin, such as we call touch, pain, and the like, but few 

 studies of these exist. Sand has shown the presence of volleys of 

 impulses in the nerves connected with muscles when the latter are 

 stretched. Proprioceptors have been demonstrated histologically in 

 the muscles of Raja. This agrees with the fact that after severance of 

 the spinal cord the swimming rhythm only continues if some afferent 

 nerves are intact. 



