NERVOUS SYSTEM OF FISHES. 181 



tricle ; the base expanding from between tlie anterior interspace of 

 the optic lobes, and the apex directed forwards and attached to the 

 roof of the cranium. Some medullary matter mingles with the mem- 

 branous walls of the conarium in the Clupeoid and Cyprinoid Fishes : 

 in some fishes there is grey matter in the conarium : in most it is 

 membranous only, as in the Lepidosiren, Sturgeon, and Shai'k : 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 

 consists 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 ventricle 

 through an interspace produced by the divarication of the main 

 lateral columns of the encephalon. * 



The fasciculi continued forwards 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 tlie optic septum {jig. 53, r.). In Osseous 

 Fishes the tAvo cerebral crura, so constituted, rarely undergo any 

 enlargement, homologous with the ' thalami,' where they form the 

 anterior boundary of the third ventricle ; but after a very brief course, 

 as ' crura cerebri,' radiate into two small subspherical 'proseucephalic' 

 masses \ of grey matter {fig. 53. r.), 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 ' rhincncephala ' {figs. 49, 50. r)., or are 

 continued forwards and outwards, as ' rhinencephalic crura' {figs. 47. 



* Is this vertical slit homologous with the encephalic ring perforated by the 

 oesophagus in Invertebrata ? 



f Influenced by the inapplicability of the term 'hemispheres' to parts which 

 are more commonly spheres or spheroids, and to avoid misconception by those 

 who attach to the word ' cerebrum' the idea of the whole brain minus ' cerebellum' 

 and ' medulla oblongata,' or who may restrict the term 'cerebral hemispheres' to 

 the super-imposed masses of the lateral ventricles in higher Vertebrata, I shall 

 apply the term 'prosencephalon' to the constant division of the brain in question, 

 and prosencephalic lobes or prosencephala to its commonly distinct moieties. It 

 is unfortunate for the student of anatomy that, in his introduction to the 

 science by the human structure, he should become acquainted with these parts of 

 the brain under the name of ' hemispheres,' as if they were two halves of an essen- 

 tially spherical whole or single organ. In most Vertebrata the homologous parts 

 are presented to our view under a form more agreeable to their true duplex nature. 



N 3 



