836 



THE NERVOUS SYSTEM 



cular tract. The inferior represents detached fibres of the pons; the superior, 

 running from the brachium of the inferior quadrigeminate body and disappearing 

 in the oculomotor sulcus, appears to be derived from the quadrigeminate bodies. 

 Since it is well developed in the cat, dog, sheep, and rabbit, but is absent or little 

 marked in the mole, it is supposed to be concerned with the optic apparatus. 



Internal structure. — Transverse sections of the mesencephalon throughout 

 are composed of — (1) a dorsal part, consisting of the lamina quadrigemina or the 

 grey substance of the corpora quadrigemina, with the strata and bundles of 

 nerve-fibres connected with them, and the abundant central grey substance 

 surrounding the aqueduct; (2) a tegmental part, consisting of the upward con- 

 tinuation of the reticular formation of the medulla oblongata and that of the 



Fig. 660. — Transverse Section 



Stratum zonale*~^ 



Nucleus of inferior^ 

 colliculus 



Aqueduct of-, 

 cerebrum 



Nucleus of mesen- i^<^/''*«^ 

 cephalic (descend- ^.^jtrW- "^^ 

 ing) root of masti-~-*-*-a/«|? ^. 

 cator ^ - 



Nucleus of trochlear -- _ ^ > 



Through the Inferior Quadrigeminate Bodies. 

 Central grey substance 



/ 

 / Nucleus of lateral lemniscus 



/' 



Lateral lemniscus (acoustic) 



Thalamo-olivary tract 



%. 





-fe' 



Medial longitudinal - 

 fasciculus 



Brachium con- 

 junctivum 



Decussation of -^ 

 brachia conjunctiva 



Posterior recess of in- «-- 



terpeduncular fossa ^"^ 





Lateral sulcus of 

 mesencephalon 



Medial lemniscus 



Substantia nigra 



Basis of cerebral peduncle 



Superficial stratum of pons 



dorsal (tegmental) portion of the pons region, to which are added the superior 

 cerebellar peduncles and the red nuclei of the tegmentum in which these peduncles 

 terminate; (3) a paired ventral part, the cerebral peduncles, each of which consists 

 of a thick, pigmented stratum of grey substance, the substantia nigra, spread 

 upon the largo, superficial, and somewhat crescentic tract of white substance 

 known as the basis of the peduncle. The cerebral p(Hluncles correspond to the 

 longitudinal or pyramidal fasciculi of the pons and medulla. Likewise the 

 lemniscus and the medial longitudinal fasciculus of the medulla and pons continue 

 through all sections of the ines(!nc(!phalon. 



'J'lie central grey substance is a continuation of the central gelatinous 

 substance of the spinal cord and the similar stratum of the medulla and that 

 which immediately underlies the ependyma of the fourth ventricle. As in the 

 spinal cord and medulla, it is largely composed of gelatinous substance. It is 

 much more abundant in th(! mesencephalon, and in sections appears as a cir- 

 cumscriljed area comparatively void of nerve-fibres. 



The nucleus of the mesencephalic root of the md.tticator nerve may likewise be traecd through- 

 out the mesencephalon. It consists of a few small bundles of fibres surrounding a thin strand 



