ENCEPHALON OF FISHES. 281 



Lepidosiren, Sturgeon, and Shark : in all it is highly vascular. 

 In the Bream the conarium shows an analogous peculiarity to 

 that of the hypophysis in the Angler, viz. in the length and 

 tenuity of its attachment ; but this consists of two distinct crura. 

 The value of the constancy of the hypophysis and conarium con- 

 sists chiefly in their marking the boundary line between the mes- 

 and pros-encephala, although they belong to the mesencephalon, 

 and are both essentially vertical prolongations of the third ven- 

 tricle through an interspace produced by the divarication of the 

 main lateral columns of the encephalon. 



The fasciculi continued forward from the parietes of the third 

 ventricle or mesencephalic basis, are principally those which may 

 be traced back through the epencephalon to the anterior and 

 lateral myelonal tracts, augmented by fibres from the grey centres 

 or lobes through which they have passed, and retaining a small 

 admixture. of post-pyramidal fibres from the optic septum, 

 fig. 184, r. In Osseous Fishes the two cerebral crura, so con- 

 stituted, rarely undergo any enlargement, homologous with the 

 6 thalami,' where they form the anterior boundary of the third 

 ventricle ; but after a very brief course, as ( crura cerebri,' fig. 178, 

 x, radiate into two small subspherical f prosencephalic ' masses of 

 grey matter, ib. P, situated anterior to the optic lobes, and there 

 in great part terminate. A few of the medullary fibres extend 

 along the base of the prosencephalon, receive a small tract of its 

 grey matter, converge to the anterior interspace of its lobes, and 

 either expand there into ' rhinencephala,' figs. 174, 175, 186, R, 

 or are continued forward and outward, as ( rhinencephalic crura,,' 

 figs. 178, 187, z } to form the olfactory lobes or ganglia, ib. R, 

 at some distance from the brain. Although the prosencephalic 

 lobes are commonly in contact with the optic lobes, yet something 

 analogous to the displacement of the rhinencephalon may be seen 

 in the prosencephalon of the Chimaera, in which the cerebral 

 crura, fig. 179, b, advance some way before they expand into the 

 prosencephala, P : in the Plagiostomes, also, the prosencephalic 

 crura, fig. 187, x } have a short independent tract in advance of 

 the optic lobes. 



The prosencephala, figs. 177, a, 180 and 182, c, 183, a, b, in 

 other figs. P, are distinguished from the optic lobes by their 

 grey pinkish exterior, and, generally, also by their fissured 

 or nodulated surface. The first of these characters must be 

 looked for in recent fish: the second is more permanent. 1 With 



1 xx. vol. iii. it may be seen in preparations of the brain of the Eel (Anguilla acuti- 

 rostris, No. 1309, B); of the Lump-fish (Cyclopterus, No. 1309, C'); of the Gurnard 

 (Trigla hjra, No. 1309, D'); and especially in this specimen of the brain of the Cod 



