MESENCEPHALON. 



MID-BRAIN, OR MESENCEPHALON, AND REGION OF THIRD 

 VENTRICLE, OR THALAMENCEPHALON (INTER-BRAIN). 



The parts of the brain next to be described are entirely covered by the cerebral 

 hemispheres. They comprise the crura cerebri and corpora quadrigemina, the optic 

 thalami with the middle commissure, and the pineal body, in addition to the 

 following structures which are seen when the brain is removed from the skull and 

 its under-surface or base is examined, viz. : the posterior perforated space, the 

 corpora albicantia, the tuber cinereum with the infundibulum and pituitary body, 

 the optic tracts and chiasma, and the lamina cinerea. Of these the corpora quadri- 

 gemina and crura cerebri are found in connection with the aqueduct of Sylvius, and 

 belong to the mid-brain, while the optic thalami and the other structures above 

 enumerated occur in connection with the third ventricle, and belong to the inter- 

 brain. 



The aqueduct of Sylvius (iter a tertio ad quartum ventriculum) is a narrow 

 passage into which the upper end of the fourth ventricle gradually narrows, and 

 which in front expands abruptly into the third ventricle. It is rather more than 



AV. 



Fig. 72. SECTIONS THROUGH THE ORIGIN OF THE FOURTH NERVE (Stilling). \ 



A, transverse section at the place of emergence of the nerve-fibres. B, oblique section carried along 

 the course of the bundles from the nucleus of origin to the place of emergence. Aq, Sylvian aqueduct, 

 with its surrounding grey matter ; IV, the nerve-bundles emerging ; IV, decussation of the nerves of 

 the two sides ; IV", a round bundle passing downwards by the side of the aqueduct to emerge a little 

 lower down; n.IV, nucleus of the fourth nerve. I, fillet; s. c. p., superior cerebellar peduncle; 

 d. V., descending root of the fifth nerve ; pi, posterior longitudinal bundle ; r, raphe. 



half an inch long. In shape it varies in different parts, being T-shaped in section 

 below (near the fourth ventricle), triangular above (near the third), and in the 

 intermediate part of an elongated oval form, but somewhat shield-shaped in the 

 region of the superior corpora quadrigemina. It is lined by ciliated columnar 

 epithelium, outside which is a thick layer of grey matter, continuous with that of 

 the fourth ventricle. Outside this central grey matter of the aqueduct, the lateral 

 and ventral parts (basal part) of the mesencephalon are composed of the thick 

 masses of the cerebral peduncles (crura cerebri), whilst the dorsum is formed by the 

 lamina quadrigemina, so called from bearing the four marnillated tubercles known as 

 the corpora quadrigemina. 



The epithelium which lines the Sylvian aqueduct is ciliated (as elsewhere in the 

 ventricles of the brain) and the attached ends of the cells extend as radiating 

 ependymal fibres through the thickness of the mid-brain to reach the surface at 

 least, this can be seen to be so in the embryo and in small vertebrates, and is 

 probably also true for all. But some of the ependymal fibres are attached to 

 neuroglia cells which occur at various levels in the course of the fibres ; they have 



