16 BRAIN AND BODY OF FISH 



an outline of the structure of the brain of a typical bony fish, and 

 we shall endeavour to do tliis without introducing any but the most 

 necessary technical terms. 



The spinal cord of a fish may be likened to the long stem of a 

 clay pipe, this, instead of being hard like baked clay, is of the con- 

 sistence of a cream cheese and of the same colour. Instead of a 

 bowl at the anterior end there are a series of thickenings of the 

 walls, which for the most part appear on the upper or dorsal aspect. 

 These swellings or lobes may be single, but more frequently there 

 are two symmetrical protuberances facing each other, which is 

 described as bi-lobed. These swellings are not necessarily associated 

 with any increase in size of the central canal, although at two points 

 of the brain, as the anterior part is called, the neural canal widens 

 out into spaces called ventricles. 



In the liinder part of the brain, which is known as the medulla 

 oblongata, another condition is noticed. In the mid-line there is a 

 longitudinal slit in the dorsal wall of the hinder ventricle, and the 

 thickenings of the walls thus seem to take place in the free margin 

 of the split tube ; there is thus left an open space or hiatus, which, 

 however, is covered in by a vascular membrane. This opening 

 out of the fourth ventricle, as it is called, is known as the rhomboid 

 fossa. 



The various lobes will now be described, and starting from the 

 simple spinal tube and passing forwards we first notice two lateral 

 protuberances (Plate 1, Fig. i), the vagal lobes. This view is drawn 

 from a specimen from which the roof of the bony cranium has been 

 removed, and the brain is looked down upon from above. These 

 lobes in the carp are pear-shaped in outline, and their anterior ends 

 are seen to be separated by a central globular lobe kno%^'n as the 

 facial lobe. In the catfish (Plate 1, Fig. ii) the vagals are globular 

 and there are two facial lobes lying in front of them. It is found 

 that in certain of the carp family that the facial lobe has a partial 

 median division, as in the gudgeon, so that the single median facial 

 of the carp and most Cyj^rinoids is due to the fusion of two lateral 

 segments. The condition in the cod (Plate 1, Fig. iii) is somewhat 

 different, as the vagal lobes do not form so prominent a swelling 

 dorsally, as we shall have occasion to note when the cod brain is 

 discussed. 



The vagal lobes receive nerves of sensation, afferent nerves 

 and also efferent nerves wliich pass outwards and are nerves of 

 motion ; so that we say that the vagal lobes have both sensory and 

 motor nerve roots. 



The facial lobe in fish is peculiar in that it receives all the sensory 



