7 HE DOGFISH 373 



end of the orbit and pass outwards and backwards to their 

 insertions into the eyeball. The obliquus superior is 

 inserted into the dorsal surface of the eyeball just in front 

 of the superior rectus ; the obliquus inferior is inserted in 

 a corresponding position in front of the insertion of the 

 inferior rectus upon the lower side of the eyeball. By the 

 contraction of various combinations of these muscles the 

 eyeball may be turned in any direction. The lower eyelid is 

 movable. The structure of the internal ear is essentially 

 similar to that of the frog. Its communication with the 

 external water and the absence of a drum have already 

 been mentioned (p. 346). Besides these sense organs, 

 which are found in all vertebrates, fishes possess a peculiar 

 system, known as the neuromast organs, which are not 

 found in any other adult vertebrates with the exception of 

 certain newts. These consist of sensory patches of the 

 epidermis containing sense cells, which bear short, stiff 

 sense hairs, and supporting cells. In the dogfish the 

 sense patches are placed at the bottom of tubes in the 

 skin, which are filled with slime or mucus. The most con- 

 spicuous of these tubes runs along the side of the body, its 

 position being marked by a rather indistinct lateral line 

 (Figs. 245, 247). It opens upon the surface of the body at 

 intervals. On reaching the head the lateral line divides 

 into two branches, which pass above and below the eye, 

 branch again, and rejoin in front upon the snout. Besides 

 this branching system of tubes there are, upon the snout, 

 others which pass straight inwards through the skin and end 

 in swellings or ampullae (Fig. 251, n.am.) which contain 

 sense patches. These can be found by pressing the skin 

 and thus squeezing the mucus out of them in little drops. 

 The neuromast organs are supplied by a special set of 

 nerve fibres, which join the. same portion of the grey 

 matter of the brain with which the fibres of the auditory 

 nerve are connected, but enter the brain by various nerves 

 (Fig. 264), of which the principal are the ophthalmic branch 

 of the seventh and the lateral line branch of the tenth nerve. 

 The function of these organs is the detection of vibrations 

 in the water of too low a frequency to be detected by the 

 ear. The latter must be regarded as a specially highly 

 developed part of the same system as the neuromast organs. 



