814 THE NERVOUS SYSTEM 



Of the cerebro-pontile paths, the frontal pontile path (Arnold's bundle) is de- 

 scribed as arising in the cortex of the frontal lobe, passing in the anterior portion of 

 the internal capsule down into the medial part of the base of the cerebral peduncle, 

 and terminating in the grey substance of the pons. The descending temporal 

 pontile path, sometimes called Turk's bundle, arises in the cortex of the temporal 

 lobe, occupies the posterior portion of the internal capsule, lies lateral in the pyramidal 

 portion of the cerebral peduncle, and terminates in the grey substance of the pons. 

 The total area in transverse section of the pyramidal fasciculi as they enter the pons 

 above is considerably greater than that which they possess as they emerge as the 

 pyramids of the medulla below. The difference is considered very appreciably 

 greater than can be explained as due to the loss of pyramidal fibres supplied to the 

 nuclei of origin of the cranial nerves lying within the level of the pons, and the ad- 

 ditional difference is explained as due to the termination within the pons of the 

 cerebro-pontile paths. 



THE CEREBRUM 



THE MESENCEPHALON 



The mesencephalon or mid-brain is that small portion of the encephalon which is 

 situated between and connects the rhombencephalon below with the prosencephalon 

 above. It is continuous with the isthmus rhombencephali, and occupies the ten- 

 torial notch, the aperture of the dura mater which connects the meningeal cavity 

 containing the cerebellum with that occupied by the prosencephalon. Its greatest 

 length is about 18 mm., and it is broader ventrally than dorsally. Its dorsal surface 

 is hidden by the overlapping occipital lobes of the cerebral hemispheres. It consists 

 of (1) the lamina quadrigemina, a plate of mixed grey and white substance which 

 goes over lateralwards and Taelow into (2), the cerebral peduncles (crura) and their 

 tegmental structures, and it contains (3), the nuclei of origin of the trochlcar and oculo- 

 motor 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 cerebral 

 peduncles. By means of the lamina quadrigemina 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 corpora quadrigemina, divided by a flat median 

 groove crossed at right angles by a transverse groove. The anterior pair of these, 

 the superior quadrigeminate bodies or superior colliculi, are larger though less prom- 

 inent than the inferior pair or inferior colliculi. Kach colliculus is continued laterally 

 and upwards into its arm or brachium. The inferior brachium proceeds from the 

 inferior colliculus, disappears beneath and is continuous into the medial genicu- 

 late body, and enters the thalamus. The superior brachium proceeds from the 

 superior colliculus, disappears between the medial geniculate body and the overlap- 

 ping pulvinar of the thalamus, and becomes continuous 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 thalamencephalon 

 rather than to the mesencephalon. The superior quadrigeminate body or superior 

 colliculus and the lateral geniculate body are a part of the optic apparatus, while the 

 inferior colliculus and the medial geniculate body belong chiefly to the auditory ap- 

 paratus (see CENTRAL CONNECTIONS < IF COCHLEAE NERVE) . Just as the cochlear nerve 

 is connected by a few fibres with the superior colliculus, so do some fibres from the opt ic 

 tract pass into the inferior colliculus. Also some fibres from the optic tract (mesial 

 root) are 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 mid- 

 line as a bridge over the opening of the aquseductus cerebri into the third ventricle. 



