426 INTRODUCTION TO CRANIATA [CH. 



The fifth or trigeminal, and seventh or facial, are most 

 interesting nerves, being sensory as well as motor. The sense- 

 organs which are supplied by the seventh nerve in the lower 

 Craniata are peculiar. These organs are scattered over the prae- 

 oral part of the body or snout and the sides of the head, and 

 are known as the mucous canals. On the snout they have the 

 shape of deep tubes swelling out at the bottom into sacs or am- 

 pullae; and on the head, of canals communicating at intervals with 

 the exterior by vertical tubes. Certain of the cells lining these tubes 

 develop blunt, freely projecting sense-hairs, recalling the character of 

 the auditory cells, whilst others secrete the mucus with which the 

 tubes are filled and whence they derive their name. It is probable 

 that the function of these organs is somewhat allied to that of the 

 ear, i.e. perception of vibrations, for it has been proved that a fish 

 deprived of its eyes is still able to guide itself along tortuous 

 passages so long as this organ remains intact, and this is explicable 

 only on the assumption that the reflected pulses of the water are 

 felt by these organs. The branches of the seventh nerve which 

 supply them are known as (a) the ophthalmic branch which runs 

 above the eye, (&) the buccal branch which runs along the roof of 

 the mouth beneath the eye, and (c) the hyomandibular which runs 

 vertically downwards behind the eye and in front of the first func- 

 tional gill-slit. The eighth or auditory cranial nerve goes to the ear, 

 and arises in such close proximity to the seventh that it may be 

 regarded as a specialised branch of it, the ear itself being very 

 possibly a highly specialised mucous canal. 



The fifth nerve supplies a series of peculiar sense-organs known 

 as end-buds. These have the form as their name implies of a bud- 

 like aggregation of cells, consisting of supporting cells intermixed 

 with sense-cells with short fine sense-hairs. These end-buds are 

 scattered over the bodies of many fish and are supplied by sensory 

 branches of the fifth nerve. In other Craniata they are confined to 

 the interior of the buccal cavity and subserve the function of taste. 

 The so-called ophthalmic branch of the fifth nerve runs over the 

 eye closely parallel to the similarly named branch of the seventh 

 but its fibres go to end-buds and to unspecialized endings in the skin 

 not mucous canals. The other divisions of the fifth and seventh 

 are distributed to the region of the mouth and to that of the first 

 gill-slit respectively. They both fork ; the upper branch of the fifth 

 goes to the upper jaw and the lower to the lower jaw, while one 

 branch of the seventh passes in front and the other behind the 



