PISCES FISHES. 



521 



(557.) The brain of an adult fish occupies but a small portion of 

 the cranial cavity ; the space between the pia mater , which invests 

 the brain, and the dura mater, which lines the skull, being occu- 

 pied by a loose cellular tissue filled with fluid : there is consequently 

 no serous or arachnoid cavity, such as exists in man. It has been 

 remarked, that the interval between the cranium and the brain is 

 considerably less in young than in mature fishes ; a fact which 

 sufficiently proves that in them the brain does not grow in the 

 same proportion as the rest of the body ; and, indeed, the size of 

 the brain is nearly equal in individuals of the same species, even 

 although the body of one be twice as large as that of the other.* 



In these, the lowest forms of Vertebrata, the brain consists of 

 several masses placed one behind the other, either in pairs or singly; 

 these masses in fact may be regarded as so many distinct ganglia, the 

 complexity and perfection of which we must expect to become gra- 

 dually increased as we proceed upwards towards mammiferous 

 quadrupeds. 



The anterior pair of ganglia (figs. 229 and 234, c ; Jig. 232, a) 

 invariably give origin to the olfactory nerves, and consequently may 

 be justly looked upon as presiding over the sense of smell. These 

 ganglia are, in fact, the representatives of those masses which in man 

 are erroneously called the " olfactory nerves ;" for even in the 

 human subject, although their real nature is obscured by the enor- 

 mous developement of other parts of the encephalon, the so-called 

 nerves are not nerves at all, but really lobes of the brain from which 

 the true nerves emanate. Fig. 230. 



(558.) The olfactory nerves of 

 fishes, derived from the lobes al- 

 luded to, vary greatly in composi- 

 tion and proportionate size : some- 

 times they are quite capillary ; 

 sometimes thick, though still sim- 

 ple ; occasionally they are double 

 or triple, and in some cases 

 are composed of numerous fibres 

 bound up in fasciculi. 



(559.) The organs of smell to 

 which these nerves are destined are 

 of very simple structure : Two 

 excavations are found near the an- 

 terior part of the snout, lined with 



* Cuv. et Val. op. cit. 



