MESENCEPHALON OF MAMMALS. 97 



testes,' ib. t, fig. G6, to the cerebellum, and thence by the resti- 

 form tracts, ib. ?*, to the myelon, s : while the transverse 

 fibres of the pons, ib. v, associate all the parts of one cerebellar 

 hemisphere in action with the other, and are intimately connected 

 and interlaced with the longitudinal fasciculi forming the crura 

 cerebri. 



If it be considered that the maintenance of the erect position 

 by Man demands unusual power of regulating and combining 

 muscular movements, whether with or without the cognisance of 



o 



the mind, and that he exercises or can exercise a greater variety 

 of modes of locomotion than any lower animal, flight alone being 

 inexecutable, the characteristic size and complexity of the human 

 cerebellum would accord with such view of its functions ; and 

 the general results of the experiments of Flourens 1 and Majendie 2 

 concur with the inferences which, in the main, may be drawn 

 from comparative anatomy (vol. i. p. 287). 



207. Mesencephalon. Part of the columnar fibres continued 

 from the epencephalon proceed directly to the prosencephalon, 

 traversing the pons, fig. 66, p, v. The olivary tracts, ib./", proceed 

 first to the mesencephalon, which likewise receives the crus, t, or 

 continuation of the restiform tract, r, after having undergone 

 cerebellar developement and connections. 



The mesencephalic basis is traversed by a forward continuation 

 of the primitive myelonal cavity- -the f iter a quarto ad tertium 

 ventriculum' which latter, fig. 105, b, is a vertical expansion 

 of the ( iter,' extending upward into the pedicle of the conarium 

 ( c pineal gland,' ib. f), and downward into that (' infimdibulum ') 

 of the hypophysis ( f pituitary gland,' ib. v). The sides of this 

 ventricular fissure are partially glued together by grey matter 

 continuous with that in the interior of the ( thalami,' and called 

 ' soft commissure ' in front of b, fig. 105. 



In the Monotremes the mesencephalic crus ( ( processus a 

 cerebello ad testes '), receiving a tract, answering to the ' fillet 

 of anthropotomy, expands into the optic lobe ( f nates,' ib.), 

 forming chiefly its exterior white layer : the primitive cavity 

 of this vesicle becomes filled with grey matter. The layer 

 ( ( valvula ') uniting the two crura becomes thickened by trans- 

 verse white fibres behind the optic lobes, and these, in higher 

 mammals, swell into a second pair of tubercles (' testes,' ib.), 

 which usually exceed the ( nates ' in breadth, but are less in 

 length ; they now T form the f corpora bigemina, or quadrigemina ' 

 of anthropotomy. The above difference in the proportion of the 



1 LV". and LXIV. 2 LVI". 



VOL. III. H 



