516 



THE NERVOUS SYSTEM. 



cerebellum becomes linked to the mesencephalon to co-ordinate the movements 

 of the body which are excited by this part of the encephalon ; and later ho\\ 

 it becomes associated with the prosencephalon, when the latter becomes respons- 

 ible for the acquisition and control of the most highly skilled actions. Foi 

 the latter purpose a great pathway of nerve -fibres is laid down to conned 

 the fore-brain with the cerebellum : the terminal stage of this connexion ii 

 situated upon the ventral (anterior) aspect of the metencephalon in the fora 

 of a great mass of transverse fibres. At one time these strands of nerve-fibrei 

 were looked upon as a bridge between the two hemispheres of the cerebellum 

 hence the name pons was applied to them. This term is now applied not onlj 



to the fibres themselves 



^* but also to the upwarc 



prolongation of the medulla 

 oblongata, to the surface oi 

 which they are applied. 



The subdivision of the 

 rest of the encephalon ink 

 mesencephalon and prosen- 

 cephalon develops later anc 

 is less fundamental thai 

 the primary demarcation 

 between them and the 

 rhombencephalon. 



The visual apparatus is 

 connected with both the 

 mid- brain and the fore- 

 brain, but at first more 

 intimately with the former 

 to which nerve pathways 

 are established to convey 

 from the spinal medulla 

 |~ and medulla oblongata sen- 

 sory impressions of touch 

 and hearing. From the 

 alar laminse of the mesen- 

 cephalon there are developed 

 four little hillocks (col- 

 liculi) corpora quadri 

 gemina to receive thest 

 FIG. 458. THE BRAIN OF A HUMAN EMBRYO IN THE FIFTH WEEK varied impressions and t< 

 (from His). enable them to influenci 



A, Brain as seen in profile. B, Median section through the same brain, the actions of the whoL 



M, Mamillary eminence ; Tc, Tuber cmereum ; Hp, Hypophysis ^odv. Special nerve path 

 (hypophyseal diverticulum from buccal cavity) ; Opt, Optic stalk ; i j j f 



TH, Thalamus; Tg, Tegmental part of mesencephalon; Ps, are laid down tr0m 



Hypothalamus ; Cs, Corpus striatum ; FM, Foramen inter- Corpora quadrigemina (Fl 



ventriculare ; L, Lamina terminalis ; RO, Recessus options ; Ri, 454") to the Spinal medull 

 Recessus infundibuli. . , , . 



to enable the mid-brain t 



control the motor nuclei of the muscles of the trunk and limbs. These are calle 

 the fasciculi tectospinales (tectum being a synonym for corpora quadrigemina). * 

 group of intercalated cells known as the nucleus ruber develops upon each side of th 

 mesencephalon for the purpose of establishing connexions between the cerebellui. 

 and the mid-brain. When an impulse passes out of the mid-brain by the tectc 

 spinal bundle to excite some movement of the body, the red nucleus provide 

 the link by which the cerebellum can co-ordinate the actions of the muscL 

 involved. By means of a fasciculus rubrospinalis it can bring its influence to be* 

 directly upon the nuclei of motor nerves in the brain and spinal medulla (Fig. 45 J 

 The prosencephalon is at first, and in some of the lower fishes remains, tl 

 most insignificant of the three brain vesicles, but in the human brain (as also : 

 that of most other vertebrates, though in varying degrees) a pair of enormo 



Ri. 



