Comparative Anatomy of the Brain. 589 



creates a series of very important distinctions from the brains of birds 

 and reptiles. Since mammals have certainly been developed from rep- 

 tiles, it is evident that the steps by which the remarkable transition was 

 made, were through animals now extinct. It will probably never be 

 possible to trace these with certainty, sinee the structure of the brain 

 could never be preserved fossil. Owen observes that if we could exam- 

 ine the brains of the Dinosaurs we might find the intermediate structures. 

 This is very likely. These animals belonged to the Mesozoic era. ( See 

 page 88. ) They were reptiles of immense size, and possessed many 

 mammalian characteristics. The long bones have a medullary cavity ; 

 the feet are short and, with the exception of the hoofed toes, like those 

 of pachyderms ; the sacrum consists of at least jive united vertebrae ; 

 the lower jaw, in some species, has lateral motion for trituration. (Dana.) 



The corpus callosum, then, is fairly inaugurated in the Rodents, In- 

 sectivores, and others of the Lissencephala. In these animals a com- 

 mencement of the septum lucidum, or wall between the anterior parts 

 of the lateral ventricles, is introduced also. It is not, however, com- 

 pleted in this sub-class, but remains elementary. In these animals the 

 hippocampi and the optic lobes are proportionally large ; the front ones, 

 the nates, being larger than the testes. The corpus striatum is small. 

 The orbits of rodents are not separated from the temporal fossa, except 

 in the cheiromys ( the aye aye ). The eyes are therefore in the brain 

 case. Behind the posterior margin of the pons varolii, on the ventral 

 face of the medulla oblongata, there is, in most of the mammals, a , 

 body of cross fibres on each side, occupying a rectangular area, and 

 named, from its shape, the corpus trapezoides. They are well defined 

 in the ornithorhynchus (fig. 302) and in the rabbit; diminish in the 

 pig and other gyrencephala, and are wanting in the apes and man, their 

 function being performed by the " arciform fibres " described in chapter 

 57, which are the homologues and derivatives of the trapezoidea. The^ 

 vermis, or vermiform process, the median lobe of the cerebellum, con- 

 stitutes the whole cerebellum in the Fishes, Birds and Reptiles. In 

 mammals it is flanked and reinforced by side lobes, (fig. 305, d. ) Rel- 

 ative to these, it is large in the lower mammals but decreases in size as 

 we ascend the scale. In the ape and man it is proportionally much re- 

 duced, and is almost entirely concealed by the overgrowth of the lat- 

 eral lobes. 



The highest of the mammal races, excluding man, are designated, 

 under Owen's brain classification, as the Gy^encephala, the name signi- 

 fying " convoluted brain. " It is descriptive of most of the brains in 

 this sub-class ; a few, however, of the smaller members of it have smooth 

 cerebrums. In the animals of this class, excepting the cetacea and the 

 elephant, the reproductive testes are in an external scrotum. Except- 



