i7o ORGANIZATION OF THE HEAD v. 15- 



visceral functions. In mammals this control is indirect, but in fishes 

 the nerves that spring from the medulla directly innervate the 

 respiratory muscles of the gills and floor of the mouth. It is no doubt 

 for this reason that the centre for the initiation of the respiratory 

 rhythm developed in the medulla. 



16. Receptor-organs of elasmobranchs 



The paired nasal sacs have much-folded walls. Water enters by a 

 single opening but this may be partly divided by a fold, making a 

 groove, which may open to the mouth. There are taste-buds scattered 

 over the wall of the pharynx. It has been shown experimentally that, 

 as in higher animals, these are receptors for sampling the food after 

 it has been brought close to the animal, whereas the nose acts as a 

 distance receptor. Smell and taste are therefore different senses for a 

 dogfish, as for us. By training fishes to discriminate between various 

 substances it can be shown that those that we should smell are 

 detected by the nose in the dogfish, but its organs of taste, like ours, 

 can discriminate only between a few qualities, including salt, sour, 

 and bitter. 



The eyes are well developed in sharks and no doubt serve as an 

 important means of finding the prey and avoiding enemies. However, 

 the retina usually contains only rods, and visual discrimination is 

 probably poor, but there are cones in Mustelus and Myliobatis. Unfor- 

 tunately details as to the functional performance of the eyes, ability to 

 discriminate shapes, &c, are scanty. Behind the retina there is often a 

 reflecting layer, the tapetum lucidum. This may be provided with 

 pigment cells, which expand in the light but contract in darkness, 

 allowing the underlying guanophores to reflect, thus increasing 

 sensitivity. The lens is spherical and very hard, as in all fishes, since 

 it must perform the whole work of refraction. It is provided with a 

 protractor-lentis muscle, presumed to produce active accommodation 

 for near vision by swinging the lens forward. The iris is peculiar in 

 those elasmobranchs that hunt by day; when it narrows it divides 

 the pupil into two slits by the descent of an upper flap or operculum. 

 The muscles of the iris are better developed in elasmobranchs than 

 in most bony fishes and the pupil makes wide excursions. The 

 sphincter iridis muscle, which narrows the pupil, works as an inde- 

 pendent effector. It is stimulated to contract by light, but its move- 

 ments are not controlled by any nervous mechanism. The radial 

 dilatator fibres, which open the pupil, receive motor-fibres from the 

 oculomotor nerve. The closure of the iris when illuminated is 



