352 AN INTRODUCTION TO ZOOLOGY 



flocculus, traversed only by five or six sulci. Between the flocculus 

 and the hemisphgerium is a lateral lobe, the paraflocculus, bent upon 

 itself and covering the dorsal surface of the flocculus. 



The ant ero- ventral surface is marked by a broad, slightly raised 

 transverse band of tissue, the pons Varolii, whose sides pass round 

 latero-dorsally to join the cerebellum. The two structures, pons 

 and cerebellum, form the major part of the metencephalon. Just 

 behind the pons lies another less conspicuous transverse area, the 

 corpus trapezoideum, which is marked in the middle line by a 

 shallow longitudinal groove, the ventral sulcus, that passes back 

 to become continuous with the ventral fissure of the spinal cord. 

 It also bears two median longitudinal bands, the pyramids, one 

 on each side of the furrow. Behind this again these fairly narrow 

 pyiamidal tracts pass backwards, and are obliquely truncated at 

 their posterior extremities, this region being termed the decussa- 

 tion of the pyramids, since at this place fibres from one side of 

 the brain pass over to the opposite side of the spinal cord. From 

 this region onwards the floor of the medulla oblongata or myelence- 

 phalon passes 'backwards without noticeable line of demarcation 

 into the spinal cord. 



The fourth ventricle, the cavity of the hind-brain, is not visible 

 externally, since it is covered by the cerebellum. It is a long 

 triangular depression on the dorsal side of the hind- brain, continuous 

 with the iter in front and the canalis centralis of the spinal cord 

 behind. The ventricular roof is very thin, being formed of ependyma 

 and the overlying pia mater. It is divided into two moieties by the 

 cerebellum. The front part, known as the anterior medullary 

 velum or valve of Vieussens, runs back from the posterior colliculi 

 under the anterior end of the cerebellum, with which it becomes 

 continuous. It contains a layer of nervous matter, thin posteriorly, 

 but thickening in front. The hinder portion of the ventricle is 

 roofed by the posterior medullary velum or choroid plexus of the fourth 

 ventricle. This is mainly non-nervous and highly vascular, and it 

 passes forwards under the end of the cerebellum, with which it 

 becomes continuous. 



The cerebellum is joined to the rest of the brain on each 

 side by three large bands of white tissue, which help to form the 

 dorso-lateral walls of the ventricle. The most anterior is the 

 brachium conjunctivum or anterior cerebellar peduncle, which runs 

 downwards and forwards beneath the corpora quadrigemina, and 

 between the two brachia stretches the anterior medullary velum, 

 which becomes fairly thick at its front margin. The middle cere- 

 bellar peduncle or brachium pontis is a stout band of white fibres 

 passing ventrally to form the main mass of the pons, which thus 



