Miscellaneous. 71 



also special, and susceptible of numerous modifications. The pecu- 

 liarities presented by it have given rise to very various interpretations, 

 often founded on deceptive analogies of form, and most of them in- 

 capable of demonstration. With the exception of some determina- 

 tions which are evident at the first glance, most of them are still in 

 the condition of simple hypotheses, or mere probabilities, or are 

 completely erroneous. In order to arrive at results such as science 

 can accept, v/e must (1) commence by referring the different parts of 

 the encephalon of Fishes to the divisions which embryogeny furnishes 

 for this brain as for those of other Vertebrata, and (2) ascertain with 

 precision the organs composing each of these divisions, and distin- 

 guish among these organs those which are fundamental and those 

 which belong to the development of higher types. 



By proceeding in this manner, tracing the series of three ence- 

 phalic regions which correspond successively to the three primitive 

 cerebral vesicles of vertebrate animals (the epencephalic, mesen- 

 cephalic, and prosencephalic), and ascertaining that the epencephalon 

 is divided into two subregions, namely, that of the postcerebrum or 

 calamus and that of the cerebellum, and that the prosencephalon is 

 also divided, forming an anterior and an intermediate cerebrum, the 

 author found no difficulty in determining, at first in a general way, 

 but afterwards in detail, the cerebral organs which are developed in 

 each of these regions in Fishes. 



For the postcerebrum, or region of the calamus, we have here two 

 pairs of small grey masses, superposed upon the roots of the fifth, 

 eighth, and ninth pairs of encephalic nerves. These httle lobules, 

 commonly known as the posterior lobes or lobi vagi, correspond to 

 the streaks of grey matter which border the fourth ventricle in the 

 higher Vertebrata, and especially the posterior pyramids, forming 

 what are called the valves of Tarin. 



The posterior brain, which usually forms a large single lobe sup- 

 ported by two lateral peduncles and emitting two anterior processes, 

 is a well-characterized cerebellum ; and by contrasting with that of the 

 Batrachia and Reptiles, but more or less resembling that of Birds, 

 it constitutes one of the features of the cerebral type of Fishes. 



The mesencephalon is concealed beneath the posterior part of the 

 prosencephalon. It is composed of tuberculiform masses seated on 

 a floor which covers a true aqueduct of Sylvius : it is here that the 

 processes of the cerebellum terminate ; and we may easily recognize 

 the tubercula geminata in the hollow grey masses which cover this 

 small ventricular region. 



The greatest difficulty of determination is presented by the pros- 

 encephalon, and especially by the posterior subregion or intermediate 

 cerebrum. This region, more complex than the others in all Verte- 

 brata, presents peculiar arrangements in Fishes ; and to decipher it 

 we must recall the constitution of the intermediate cerebrum in the 

 higher Vertebrata. It is at once the peduncular region, the region 

 of the third ventricle, of the cerebral nucleus — or at least of the 

 fundamental portion of that nucleus, which is composed of the pedun- 

 cular fasciculi, the optic lobes, and the striated bodies. 



In Fishes we readily detect this peduncular region, composed 



