18 BRAIN AND BODY OF FISH 



position anterior to the facial lobes there is a bilateral area receiving 

 sensory fibres from the skin, this can be seen in the drawing of 

 the cod's brain, and it is known as the somatic-sensory or fifth lobe ; 

 it is not apparent in the carp or catfish when seen from above, but 

 it is clearly evident in sagittal sections. 



The next prominent lobar swelling is a large central protuberance 

 smaller in the carp than the catfish, and forming a tongue-like body 

 overlapping the medulla oblongata in the cod ; this is known as 

 the cerebellum. 



From either side of the cerebellum leading to the sides of the 

 somatic-sensory or fifth lobes are marked rounded prominences, 

 varying much in size in different species, and known as the acoustic 

 tubercles. Anterior to the cerebellum are two large globular swell- 

 ings known as the optic lobes, which receive the large optic nerves 

 on their ventral aspect, and at the anterior end of each oj^tic lobe 

 are smaller lobes which form the primitive end-brain into which 

 enter the olfactory nerves, leading from the olfactory bulbs, or are 

 in continuity with the olfactory lobes. 



As this end-brain puzzles even a trained biologist, we will 

 further explain that the end-brain in which the olfactory centres 

 lie consists generally of two sections, the anterior being the olfactory 

 bulbs which are the terminations of the olfactory nerves, and the 

 posterior the larger, the jjrimitive end-brain, in wliich the walls 

 of the neural tube are thickened. The olfactory bulbs usually lie 

 close to the end-brain as is seen in the drawing of the brain of a 

 plaice, but in some bony fish the nasal sacs are far removed, as is 

 seen in Plate 1, Fig. I, the drawing of the brain of a carp, where the 

 bulbs appear separated by the pallium, and the olfactory stalks 

 from the primitive end-brain. 



If we now look at Plate 2, which is a drawing of the brains of 

 a carp and a cod viewed sideways when a sagittal section is made 

 through one half of the brain we shall be able to visualise the above 

 described organs in another dimension. 



In man the sensory columns of the spinal cord lie posteriorly, 

 but these columns become dorsal in fish and so we find the sensory 

 centres of the brain appearing on the dorsal aspect in fish. 



If we examine the diagrams of Plate 2 it will be seen that in 

 both cod and carp the tecta optica which form the prominences 

 of the optic lobes are similar, but the forward projection of the base 

 of the cerebellum, known as the valvula cerebelli, is bigger in the 

 carp than the cod and further in the carp tends to separate the 

 tecta of either side. 



When the medulla oblongata of the cod is examined, the sldn 



