530 PISCES FISHES. 



its position and singleness. In the Perch its form is that of a 

 blunted cone, with the summit directed slightly backward, but the 

 shape and relative dimensions of this part of the brain are extremely 

 variable. It consists, in fishes, only of the central portion (pj'o- 

 cessus vermiformis), so that there are neither lateral lobes nor pons 

 Varolii : its surface is composed of cineritious substance, and in its 

 centre is a ramified medullary axis containing a ventricle that com- 

 municates with the fourth. 



One very remarkable feature in the structure of the encephalon 

 of fishes is the existence of supplementary lobes (Jig. 234, g) 

 placed behind the cerebellum, which sometimes are united by a 

 commissure : occasionally, as in the Trigla, there are as many as 

 five pairs of such supplementary masses ; but probably, instead of 

 regarding these as belonging to the brain, it would be more proper 

 to consider them as being merely the first ganglia composing the 

 spinal cord enormously developed in proportion to the importance 

 of the nerves which they give off to the pectoral fins. 



(574.) The spinal nerves of fishes arise by double roots from the 

 sides of the medulla spinalis, which generally extends from one 

 end of the canal formed by the superior vertebral arches, to the 

 other. The posterior roots are dilated into ganglia soon after 

 their origin, but the ganglia are extremely minute. The spinal 

 cord of the Moon-fish (Orthagoriscus Mold) is, however, an 

 exception to the usual conformation : in this remarkable fish the 

 spinal ganglia are all collected into a stunted mass placed imme- 

 diately behind the brain ; and from this all the spinal nerves are 

 given off, in the same manner as those forming the cauda equina 

 in the human subject. 



(575.) The Sympathetic system in the creatures we are now 

 examining is of very small size, when compared with that met 

 with in the higher Vertebrata ; nevertheless, it occupies the usual 

 position, and communicates as in man with the commencements 

 of the spinal nerves. 



(576.) There are few subjects more calculated to arrest the 

 attention of the physiologist than the progressive developement of 

 the generative system in the Vertebrate classes ; and it is not a 

 little interesting to watch the gradual appearance of additional 

 organs, both in the male and female, as we advance upwards in 

 the series of animated beings from the cold-blooded and apathetic 

 fishes. In its simplest condition, the whole generative apparatus, 

 even of a vertebrate animal, is in both sexes merely a capacious 



