EMBRYOLOGY AND COMPARATIVE ANATOMY OF BRAIN. 27 



selves with the deep marrow. This is the corpus quadrigermnum 

 posticum. In mammals, in whom tlie anterior portion of the 

 roof remains relatively small, this posterior quadrigeminal body 

 reaches nearly the size of the anterior one. In all the figures 

 it is marked by shading. 



Not only in the bony fishes, but also in birds, the roof of 

 the mid-brain has undergone special development. The simple 

 hollow hemisphere grows outward and downward on both sides, 

 so that the lengthened roof closes around the lateral portion. 

 Inasmuch as this condition, peculiar to birds, would not be 

 visible in a sagittal section, the mid-brain in Fig. 16 is not 

 divided, but is left so as to show it as it appears in the uncut 

 brain. 



The base of the mid-brain is formed by masses of fibres 

 which come from the fore- and inter- brain to pass farther back. 

 To these masses are added the fibres springing from the roof of 

 the mid-brain, and, lastly, there are found in this situation a 

 number of nuclei from which bundles of fibres arise, which in 

 part pass to the cerebellum, and in part reach the surface of the 

 brain as peripheral nerves (oculomotor, trochlear). One con- 

 dition, which is rudimentary in birds, is more developed in 

 mammals. Many fibres from the cortex of the hemisphere^ are 

 massed together and lie ventrad of the fibres at the base of the 

 mid-brain. This mass, called the pes of the cms cerebri, or 

 crusta, is very strongly developed in primates and in man. In 

 such brains we call all that lies dorsad of it and beneath the 

 roof of the mid-brain the tegmentum. Fishes, reptiles, and 

 amphibians possess only the tegmental tracts ; the fibres of the 

 crusta are wanting, because in them no fibres pass downward 

 from the cortex. The majority of the fasciculi of the crusta and 

 tegmentum pass on to the base of the cerebellum and medulla 

 oblongata, where many terminate. A part pass into the roof of 

 the hind-brain. 



This roof, which is continuous in front with the layer of 

 the corpora quadrigemina, and behind, through the intervention 



