40 BRAIN AND BODY OF FISH 



outer layer varying very much in thickness, which has a uniform 

 consistence, and stains with difficulty, called the stratum moleculare 

 {see Plates 4 and 5, Figs, v and vi). In Fig. v the cerebellum is 

 separated from medulla, but in the last section, Fig. vi, the cere- 

 bellum is no longer separated from the medulla and the tissue that 

 unites it has two large lateral prominences, the acoustic tubercles, 

 which have a granular structure rather finer than the granular layer 

 of the cerebellum, but also staining deeply. Below on either side 

 Plate 4, Fig. vi, are seen the lateral line nerves entering these 

 lobes. 



We have deferred from speaking of the lateral line system till 

 now as it is a controversial subject. There is present in fishes a 

 system of small sensory canals widely distributed under the skin. 

 These contain sensory organs somewhat similar to those of the semi- 

 circular canals of the internal ear and their functions are probably 

 intermediate between those of the organs of touch in the skin and 

 those of the internal ear, responding to water vibrations of low 

 frequency, and probably in the orientation of the body in space. 

 These are the lateral line canals and we are all familiar with the 

 lateral line in fish, which is so clearly seen running along the side 

 of fish nearly midway between the dorsal and ventral margins and 

 often made more obvious by being pigmented. Similar canals are 

 also found on the head. These canals are supplied by special roots 

 of the following cranial nerves which aU finally enter the acoustic 

 tubercle or acoustico-lateralis lobe ; these are the seventh or facial 

 nerve, the ninth or glossopharyngeal nerve, and the tenth or vagal 

 nerve. These, together with the eighth or acoustic nerve, are asso- 

 ciated with the reception of vibatory sensations of a greater range 

 from those of hearing proper to slow vibrations such as are felt by 

 the skin of man and also by what is known as bone conduction. 



It will be our aim to see whether the methods of comparative 

 anatomy may not help to unravel the central areas which must 

 presumably be associated with these various functions, and with 

 this object in view we must draw the attention of the reader to an 

 area of small cells interpersed with transverse nerve fibres which 

 appears at the base of the cerebellum just before its free portion 

 joins laterally with the medulla. This area is known as the central 

 acoustic area and at times forms a definite lobe. It is well shown 

 in Plates 4 and 5, Fig. v, and it will be observed that it is more 

 prominent in Engraulicypris than in the roach. In the bleak it 

 is still more prominent. Fibres pass to this area from the eighth 

 nerve, and there are also fibres passing from it to the acoustico- 

 lateralis area. 



