816 



THE NERVOUS SYSTEM 



ous openings for the passage of blood-vessels. It is divided by a shallow median 

 groove and is marked off from the inner surface of each peduncle by the oculomotor 

 sulcus, out of which emerge the roots of the oculomotor nerves. The ventral sur- 

 face of each peduncle is rounded and has a somewhat twisted appearance, indicating 

 that its fibres curve from above medianwards and inwards. Sometimes two small, 

 more or less transverse bands of fibres may be noted crossing the peduncle an in- 

 ferior, the tamia pontis, and a superior, the transverse peduncular tract. The inferior 

 represents detached fibres of the pons; the superior appears to be derived from the 

 quadrigeminate bodies. Since it is well developed in the cat, dog, sheep, and rab- 

 bit, 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 continuation of the reticular 

 formation of the medulla oblongata and that of the dorsal (tegmental) portion of the 

 pons region, to which are added the superior cerebellar peduncles and the red nuclei 



FIG. 608. DIAGRAM OF LATERAL VIEW OF MESENCEPHALON AND ADJACENT STRUCTURES. 



(After Gegenbaur, modified.) 



PULVINAR OF TIIALA3IUS 



LATERAL GENICVLATE 

 BODY 



CEREBRAL PEDUNCLE 



PONS 



OLIVE 



SPIPffTSIS 



MEDIAL GEXKTr.ATE BODY 



CORPORA Ql'ADRIGEMIXA 



LA TER.l I. LSMVISCTJS 



SI' PERU 1 1! ( /;/. F.RELLA R 



PEDUNCLE 

 MI i in i.r. <'i- iti-:r,i-:i.LAR 



PEDUNCLE 

 1SFERIO I: I 'A'/.' Eli ELL A R 



PEDUNCLE 



' ' 



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 large, superficial, and somewhat 

 crescentic tract of white substance known as the basis of the peduncle. 



The cerebral peduncles 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 mesencephalon. 



The central grey substance is a continuation of the central grey substance of the 

 spinal cord and the similar stratum of the medulla and that which immediately under- 

 lies 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 the mesen- 

 cephalon, and in sections appears as a circumscribed area comparatively void of 

 nerve-fibres. 



The nucleus of the mesencephalic root of the trigtminus may likewise be traced 

 throughput the mesencephalon. It consists of a few small bundles of fibres surround- 

 ing a thin strand of nerve-cells of the motor type which give origin to its fibres. It 

 courses downwards close to the lateral margin of the central grey substance, and is 

 quite small at its beginning in the extreme superior part of the mesencephalon, but 

 as it descends towards the exit of its fibres from the pons as a part of the motor root 



