834 



THE NERVOUS SYSTEM 



peduncles (crura) and their tegmental structures, and it contains (3), the nuclei 

 of origin of the trochlear and oculomotor nerves. It arises from thickenings of the 

 walls of the middle cerebral vesicle of the embryo, the lamina quadrigemina 

 arising from the dorsal or alar lamina of this portion of the neural tube, while the 

 basal lamina thickens to form the nuclei of the nerves, the substantia nigra, etc., 

 and b}' the ingrowing of the cerebral peduncles. By means of the lamina quad- 

 rigemina roofing it over, the neural canal throughout the mesencephalon retains 

 its tubular form and is known as the aquaeductus cerebri (Sylvii), connecting the 

 cavity of the fourth ventricle below with that of the third ventricle above. 



External features. — Dorsal surface. — The lamina quadrigemina shows four 

 well-rounded elevations, the quadrigeminate bodies [corpora quadrigemina], 

 divided by a flat median groove crossed at right angles by a transverse groove. 

 The anterior pair of these, the siiperior quadrigeminate bodies [colliculi], are 



Fig. 658. — Dorsal, Surface of Mesencephalon and Adjacent Parts. (After Spalteholz.) 



Epiphysis (lifted) 

 Posterior commissure 



Superior colliculus 



Brachium quadri- 

 geminum superius ^^ 

 Brachium quadri- 

 geminum Inf erius ^^^ 



Medial genicu- 

 late body 



lateral genicu- 

 late body 



Cerebral peduncle ™— 

 Inferior colliculus 

 Frenulum of anterior med 

 ullary velum 



Trigone of lemniscus 

 Trochlear nerve 



Brachium conjunctivum 

 Lateral filaments of pons 



Anterior tubercle of 

 ,/" thalamus 



Taenia chorioidea 

 Z^amina affixa 



Stria terminalis 

 of thalamus 



" Quadrigeminate bodies 

 Lateral sulcus of 

 mesencephalon 

 Trochlear nerve (cut oflf) 



' Trigeminus 

 — Lingula of vermis 

 --- Vinculum of lingula 

 --• Brachium of pons 



-Cerebellum (cut) 



larger though less prominent than the inferior pair or inferior colliculi. Each 

 colliculus is continued laterally and upward into its arm or brachium. The 

 inferior brachium proceeds from the inferior colliculus, disappears beneath and is 

 continuous into the medial geniculate body, and enters the thalamus. The supe- 

 rior brachium proceeds from tlie superior colliculus, disappears between the medial 

 geniculate body and the overlapping pulvinar of the thalamus, and becomes con- 

 tinuous with the lateral geniculate body and thus with the lateral root of the optic 

 tract. 



The geniculate bodies are rounded elevations of grey substance which arise as detached 

 portions of the thalami, and therefore belong to the thalaniencephalon rather than to the 

 mesencephalon. The superior quadrigeminate body or superior colliculus and the lateral gen- 

 iculate body are a part of the optic apparatus, while the inferior colliculus and the medial genicu- 

 late body belong chiefly to the andilnry apparatus (see Central Connections of Cochlear 

 Nerve). Just as the tcjrininal cochlear nuclei are connected by a few fibres with the superior 

 colliculus, so do some fibres from the optic tract pass into the inferior colliculus. Also some 

 fibres form the optic tract (mesial root) jire said to terminate in the medial geniculate body. 

 Resting in the broadened medial groove between the superior quadrigeminate bodies lies 

 the non-nervous epiphysis or pineal body. This also belongs to the thalamencephalon. 

 Under the stem of the epiphysis is a strong transverse band of white substance crossing the 



