Chap. XL] THE BRAIN AND NERVES. 217 



the spinal cord are continued. On either side of the dorsal 

 fissure are the dorsal pyramids ; on either side of the ventral 

 fissure are the ventral pyramids (v. p.). The dorsal pyramids 

 diverge at the fourth ventricle. In the rabbit there is further 

 differentiation. External to the ventral pyramids, is on either 

 side, an olivary tract, and (anteriorly) the trapezoid body (c. .), a 

 band of transverse fibres. External to the dorsal pyramids, 

 which form but narrow areas, are the restiform bodies. In the 

 medulla oblongata the white matter is external and the grey 

 internal. 



2. Metencephalon. 1 Here the sides and floor of the canal are 

 thickened. In the frog and pigeon they are not well differentiated 

 from the medulla ; but in the rabbit they are differentiated into 

 strong bands of transverse fibres forming ihepons Varolii (p. va.). 

 The roof undergoes special development, giving rise to the cere- 

 bellum (cb.), in which the white matter is internal and the grey 

 external. In the frog this is a small tongue-shaped band over- 

 hanging the anterior part of the fourth ventricle. In the pigeon 

 it is large, uni-lobed, overlying the fourth ventricle behind, and 

 abutting against the cerebral hemispheres in front. It is marked 

 with ridges and grooves. At each side there is a small elevation, 

 the flocculus (fl.). It is connected with the thickened floor of 

 this region by strong peduncles. A delicate sheet of nervous 

 tissue, the mlve of Vieussens, connects it with the optic lobes 

 (op. I.). In the rabbit the cerebellum is large and tri-lobed. 

 Each lateral lobe has externally a large flocculus. A valve of 

 Vieussens connects it anteriorly with the testes (te.), and on 

 each side this band is thickened to form the anterior peduncles ; 

 middle peduncles pass downwards to the pons Varolii ; posterior 

 peduncles are continued into the restiform bodies of the medulla. 



(B.) Mid-Brain 3. Mesencepkalon. In the frog the floor is 

 thickened, but not further differentiated. The roof gives rise 

 to large optic lobes (op. /.), which are apposed in the middle line. 



1 Some authors apply the term metencephalon to that part of the brain which 

 has been described as myelencephalon, applying the term epencephalon to the 

 cerebellum. 



