84 



ANATOMY OE VERTEBRATES. 



is small in the olivary tracts, and does not form a i corpus denta- 

 tum.' The pons is flat, it forms a narrow transverse band in the 



51 



Side view and base of brain, Ornithorhynchus. 



Base of brain, Echidna. 



63 



Ornithorhynchus, fig. 51, c; these fibres cover a greater antero- 

 posterior extent of the macromyelon in the Echidna, and give 

 the pons a triangular form. 



In the Opossum the pons, fig. 53, b, is reduced almost to the 

 proportions of that in the Ornithorhynchus ; 

 the prepyramidal, d, and olivary tracts are 

 similar, and the latter are crossed by as well- 

 marked a trapezoid arrangement of trans- 

 verse fibres, c. 



The prepyramidal tracts come to the sur- 

 face at a greater distance from the pons, in 

 most Mammals, than in Man, and thus 

 resemble more the postpyramidal tracts; 

 this character is shown in the Horse, fig. 

 54, Dolphin, fig. 60, b, and Baboon, fig. 62. 

 In the anthropoid Apes, the proportions of 

 the prepyramids (fig. 112, Orang) approach 

 those in Man, and the arciform disposition 

 of the superficial layer of crossing fibres begins to prevail, and 

 to allow the olives, which are likewise here more prominent, 

 to come into view. Although the olives are less prominent 

 in Delphinus than in the Apes, they are equally uncovered 

 by the trapezoid fibres : and show internally the arrangement 



Base of brain, Didelphys. 



