182 LECTURE vni. 



51, 55. z), to form tlie olfactory lobes, or ganglia (ib. r), at some 

 distance from the brain. Although the prosencephalic lobes are com- 

 monly in contact with the optic lobes, yet something analogous to the 

 displacement of the rhinencephala may be seen in the prosence- 

 phala of the Polypterus and Lepidosiren, in which the procense- 

 phalic crura advance some way before they expand into the prosen- 

 cephala : in the Plagiostomes, also, the prosencephalic crura {^g. 55, x) 

 have a short independent course in advance of the optic lobes. 



The prosencephala are distinguished from the optic lobes by their 

 grey pinkish exterior, and, generally, also by their fissured or nodu- 

 lated surface. The first of these characters must be looked for in 

 recent fish : the second is more permanent, and may be seen in the 

 preparations of the brain of the^el(Atiguilla acutirostris, No. 1309,B.); 

 of the Lump-fish ( Cyclopterus, No. 1309, C.) ; of the Gurnard ( Trigla 

 lyra, No. 1309, D') ; and especially in the specimen of the brain of 

 the Cod (No. 1309), which Hunter truly, though briefly, describes as 

 follows: — " The cerehrum fissured ; the cerebellum a long projecting 

 body, also fissui-ed in a less degree ; the nates two projecting bodies : 

 the optic nerves decussate one another." This is the earliest recog- 

 nition of the homology of the optic lobes with the anterior of the 

 bigeminal bodies of the human brain. With regard to the ' cerebrum' 

 of the Cod, a median tract or convolution is marked oif by a longi- 

 tudinal fissure, which extends along the back of each prosencephalon, 

 defining also a posterior and inferior convolution ; the median con- 

 volution is vertically fissured on its inner side. In the Amblyopsis 

 {Jig. 50. p) it is cleft anteriorly ; and here, as in most fishes, the 

 median longitudinal tract is the most constant subdivision of the 

 prosencephalic superficies. 



The large elongated prosencephala are smooth in Polypterus and Le- 

 pidosiren {Jig. 54. p), and in the still more developed confluent mass 

 ■54 in the Sharks {Jig. 55. p) ; the pro- 



sencephala are, also, smooth in the 

 Myxines, where they are relatively 

 p^^T'iai^^^- (|i|j j^^i^^- smallest. The comparative ana- 

 Brain of Lopidosiren.'^ tomists, who have failed to recog- 

 nise the true homology of the pro- 

 sencephalon in Osseous Fishes, appear to have been misled chiefly by 

 its small proportional size, Avhich is commonly that exhibited in these 

 preparations of the brain of the Cod (Jig. 53, p), the Carp, and the Globe- 

 fisli * ; in some species the prosencephalon is still smaller, as in the 

 Gar-fish, the Herring (Jig. 52, p), or the Lump-fish. The prosencephalon 



* The preparations exhibited and here alluded to are those numbered 1309, 

 1309 A, 1309 »). 



