NERVOUS SYSTEM OF FISHES. 187 



creasing capacity of the cranium is made by a concomitant develop- 

 ment of the light cellular arachnoid, which has the further advantage 

 of regulating the specific gravity of the head. 



As the branchial respiration is a peculiarly active and important 

 function in Fishes, and is served by an extraordinary apparatus of bony 

 or gristly arches with their muscles, we may associate therewith the 

 peculiar development and complexity of the medulla oblongata, as 

 the centre of the vagal or respiratory nerves. The Carp and other 

 cyprinoid Fishes, which have not the mechanical modifications for 

 retaining water in contact with the gills, so characteristic of the 

 Apodal, the Lophioid, and Labyrinthibi'anch fishes, are remarkable, 

 nevertheless, for their tenacity of life out of water ; and the peculiarly 

 developed vagal lobes in them may relate to this maintenance of the 

 power of the respiratory organs during a suspension of their natural 

 actions. 



The extensive gradation of the cerebellum between the extremes 

 of structure presented by the Myxine and the Shark throws, as 

 might be expected, more direct light upon its function. With regard 

 to this, two views have been taken. According to one it is the 

 organ of amativeness ; according to the other it is the seat of the 

 muscular sense, or the regulator of voluntary motion. Many expe- 

 riments in which the cerebellum has been mutilated or removed 

 in warm-blooded animals support the idea of its intimate relation 

 with the locomotive powers. But to the conclusions from these ex- 

 periments has been objected the possibility of the convulsive mus- 

 cular phenomena having arisen from the stimulus on tlie remaining 

 centres, occasioned by the mutilation or destruction of the one in 

 question ; and it may well be doubted Avhether Nature ever answers 

 so truly when put to the torture, as she does when speaking volun- 

 tarily through her own experiments, if we may so call the ablation 

 and addition of parts which comparative anatomy offers to our con- 

 templation. 



If, in reference to the sexual liypothesis of the cerebellum, we 

 contrast the Lamprey with the Shark, we shall be led, by the much 

 larger proportional size of the generative organs in the lower car- 

 tilaginous Fish, and by the observed fact of the male and female 

 Lampreys entwining or wreathing themselves entirely about each 

 other, mutually aiding in tlie expulsion of their respective generative 

 products and so absorbed in the passion as to permit tliemselvcs to 

 be taken out of the water and replaced there without interruption 

 of the act, to expect a larger cerebellum in the Lamprey than in 

 the Shark. But the reverse of this is the fact : the Lamprey has 

 the smallest, and the Shark the largest, cerebellum in the class of 



