PROSENCEPHALON OF MAMMALS. 101 



developed in the hippocampal fold,, and which form the main part 

 of the hippocamp and its anterior extension. 1 



This fold and its concomitantly developed longitudinal and 

 transverse or arched fibres, constitute a great and abrupt dis- 

 tinction and rise in structure in the Mammalian brain as com- 

 pared with the Avian one, and indicate that birds are an offshoot 

 from the lower Ovipara, forming a branch apart. 2 



In Ornithorhynchus the postero-inferior parts of the hemispheres 

 are brought into connection with the antero-internal parts by 

 the longitudinal fibres, while the antero-internal parts of the 

 hemispheres are connected with each other through the transverse 

 fibres at the approximated anterior ends of the folds, where the 

 stratum connecting those ends together, and radiating the fibres 

 upon the inner surface of the anterior lobes of the hemispheres, 

 and over the inner wall of the ventricle, is thickest. 3 



The greater part of the hemispheral cavity or ventricle is 

 overarched in Lyenccphala by the inner leaf of the hippocam- 

 pal fold, and its developements called ' trenia hippocampi ' and 

 6 fornix.' The transverse fibres connecting the taenia hippo- 

 campi and terminating that body anteriorly in Lyencephala, are 

 carried, in the ascending Mammalian series, by the growth of 

 the hemispheres anterior to them, as it were by a movement 

 of rotation, from before upward and backward, until, in Man, 

 they become the ' psalterial fibres ' which connect the posterior 

 ( genu ' of the corpus callosum with the ( trenia hippocampi,' these 

 being compared to the ' frame ' and the transverse fibres to the 

 6 strings ' of the harp, by the old anthropotomists. The super- 

 addition of cerebral matter above and anterior to c, figs. 69, 73, 

 is associated with transverse commissural fasciculi, progressively 

 added, from behind forward, and now overarching the lateral ven- 

 tricles, and fulfilling all the functions, relations, and definitions of 

 the anthropotomical 'corpus callosum,' figs. 78, /, and 123, c. 

 Its hind part is embraced by the ( callosal convolution,' ib. o. 4 



1 These fibres are shown at x, fig. 4, pi. vii. LXX'., which gives a view of the hippo- 

 campal fold from the ventricular or 'lateral' side, as ' part of a thin stratum of medul- 

 lary fibres arching over the hippocampus major, and continued therefrom into the 

 internal wall of the ventricle,' p. 95. 



2 If we could examine the brains of Dinosauria or Dicynodontia, the actual gap in 

 the series of cerebral structures might be better filled. 



3 From this point in the lowest (Lyencephalous) mammals, as in the embryo of 

 the highest, the growth of the great supraventricular body of transverse commissural 

 fibres forming the ' corpus callosum ' begins : ' Anterior fibres of the " tasnia hippo- 

 campi v continued into the anterior lobes of the hemispheres.' LXX'. p. 95, pi. vi. 

 figs. 4 and 6, o'; and pi. vii. fig. 4, x. 



4 The part marked B in the Echidna has become the part marked N in Man. Pis. 

 xxxvi. and xxxviii. of XLIII". 



