24 NERVOUS SYSTEM OF VERTEBRATES. 



nerve formed by these rootlets supplies the rectus externus muscle 

 of the eye-ball. 



The metencephalon is a short segment of the brain which ven- 

 trally appears to be merely a continuation forward of the myelen- 

 cephalon, but dorsally is sharply distinguished by the possession 

 of a massive roof instead of a choroid plexus. This massive roof 

 is the cerebellum. It is large in all true fishes and in some selachians 

 it is the most prominent part of the whole brain. In bony fishes 

 it projects inward also, encroaching upon the fourth ventricle 

 and largely filling the cavity of the mesencephalon. In cyclo- 

 stomes, dipnoans and amphibians the cerebellum is very small, but 

 in reptiles, birds and mammals it becomes progressively larger 

 and more important. Its size is evidently correlated with the 

 activity of the animal and with the number and importance of the 

 cutaneous sense organs. In all vertebrates the cerebellum consists 

 fundamentally of an arch of gray matter covered externally by a 

 fiber layer which forms a commissure dorsally. The two pillars 

 of the arch are continuous with the somatic sensory column of 

 the medulla oblongata. In mammals and man the ventral wall 

 of the metencephalon is greatly thickened and forms a ventral 

 protuberance known as the pons Varolii. 



That portion of the dorsal wall of the brain which connects 

 the cerebellum with the mesencephalon is thin in most vertebrates 

 and is known as the velum medullare anterius. This velum 

 undergoes various modifications which are discussed in a later 

 chapter (see p. 171). The fourth cranial or trochlearis nerve, which 

 crosses with its fellow in the velum and emerges from the brain 

 between the cerebellum and mesencephalon, is reckoned with the 

 cerebellar segment. It supplies the superior oblique muscle of 

 the eye-ball. 



The mesencephalon or midbrain is perhaps the most constant 

 portion of the brain in vertebrates. Its ventral and lateral walls 

 are always massive, its cavity a narrow canal, the aqueduct of 

 Sylvius. Its dorsal wall is less thick and is divided by a longi- 

 tudinal furrow into lateral portions which are known as the optic 

 lobes, because they serve as the place of ending of the fibers of the 

 optic tract coming from the retina. In mammals the lateral 



