132 MAN AND THE LOWER ANIMALS. n 



being once thus knit together, the progress of 

 cerebral complexity is traceable through a com- 

 plete series of steps from the lowest Rodent, or 

 Insectivore, to Man; and that complexity consists, 

 chiefly, in the disproportionate development of the 

 cerebral hemispheres and of the cerebellum, but 

 especially of the former, in respect to the other 

 parts of the brain. 



In the lower placental mammals, the cerebral 

 hemispheres leave the proper upper and posterior 

 face of the cerebellum completely visible, when 

 the brain is viewed from above; but, in the higher 

 forms, the hinder part of each hemisphere, sepa- 

 rated only by the tentorium (p. 136) from the an- 

 terior face of the cerebellum, inclines backwards 

 and downwards, and grows out, as the so-called 

 " posterior lobe/' so as at length to overlap and 

 hide the cerebellum. In all Mammals, each 

 cerebral hemisphere contains a cavity which is 

 termed the " ventricle "; and as this ventricle is 

 prolonged, on the one hand, forwards, and on the 

 other downwards, into the substance of the hemi- 

 sphere, it is said to have two horns or " cornua, 

 an " anterior cornu," and a " descending cornu 

 When the posterior lobe is well developed, a third 

 prolongation of the ventricular cavity extends into 

 it, and is called the " posterior cornu." 



In the lower and smaller forms of placental 

 Mammals the surface of the cerebral hemispheres 

 is either smooth or evenly rounded, or exhibits a 





