NERVOUS SYSTEM OF THE VERTEBRATA. 351 



the portion of the ventricle contained in it is prolonged into two 

 lateral outgrowths. 



Mikhxcho-Maclay, from liis observations on the brains of adult Elasmo- 

 branchii, was led to regard what is here called the cerebellum as identical 

 with the mid-brain, and the true mid-brain as part of the thalamencephalon. 

 Miklucho-Maclay was no doubt misled by the large size of the cerebellum, 

 but, as we have seen, this body does not begin to be conspicuous till late in 

 embryonic life. 



The mid-brain and thalamencephalon (according to the ordinary inter- 

 pretations) have in the embryo of Elasmobranchs exactly the same relations 

 as in the embryos of other Vertebrates; so that the embryolo^ical evidence 

 appears to nie to be conclusive against Miklucho-Maclay's view. 



In Birds the cerebellum attains a very considerable development 

 (fig. 250, cbl), consisting of a folded central lobe with an arbor vita^, 

 into which the fourth ventricle is prolonged. There are two small 

 lateral lobes, apparently equivalent to the flocculi. Anteriorly the 



Fig. 250. Longitudinal section thuough the brain of a Chick of ten days. 



(After Mihalkovics. ) 

 him. cerebral hemispheres ; alf. olfactory lobe ; alf^. olfactory nerve; (f0. corpus 

 striatum; oma. anterior commissure; chd3. choroid plexus of the third ventricle; 

 vin. pineal gland; cmp. posterior commissure; trm. lamina terminalis; chm. optic 

 chiasma; inf. infundibulum; hph. pituitary body; bgm. commissure of Sylvius (roof 

 of iter a tertio ad quartum ventriculum) ; vma. velum medullas anterius (valve of 

 Vieussens); cbl. cerebellum; chdi. choroid plexus of the fourth ventricle ; obti. roof 

 of fourth ventricle; obi. medulla oblongata; pm. commissural part of medulla; inv. 

 sheath of brain ; Ms. basilar artery; crts. internal carotid. 



cerebellum is connected with the roof of the mid-brain by a delicate 

 membrane, the velum medullge anterius, or valve of Vieussens (fig. 

 250, vma). The pons Varolii of Mammalia is represented by a small 

 number of transverse fibres on the fioor of the hind-brain immediately 

 below the cerebellum. 



In Mammalia the cerebellum attains a still greater development. 

 The median lobe or vermiform process is first developed. In the 

 higher Mammalia the lateral parts forming the hemispheres of the 

 cerebellum become formed as swellings at the sides at a considerably 



