842 THE NERVOUS SYSTEM 



enormously developed and in most of the mammals they are relatively larger and 

 appear more complicated in structure than in man. They are concerned almost 

 wholly with the visual apparatus, mediating most of the reflexes with which it 

 is concerned. 



The nucleus of the superior colliculus is of somewhat greater bulk than that of the inferior. 

 It is capped by a strong stratum zonule (fig. 662), which has been described as composed chiefly 

 of retinal fibres, passing to it from the optic tract by way of the superior brachium, but, since 

 Cajal found in the rabbit that extirpation of the eye is followed by very slight degeneration of 

 the stratum zonale, it is probable that it is composed of other than retinal fibres — possibly 

 fibres from the occipital cortex and fibres arising within the nucleus itself. The nucleus is 

 separated from the central grey substance by a weU-marked band of fibres, the stratum album 

 profundum. This contains fibres from two sources: — (1) fibres from the lateral lemniscus 

 which approach the nucleus from the under side, some to terminate within it, others to cross 

 to the nucleus of the opposite side; (2) fibres which arise within the nucleus and course ventrally 

 around the central grey substance, both to terminate in the nucleus of the oculomotor nerve and 

 to join the medial longitudinal fasciculus and pass probably to the nuclei of the trochlear and 

 abducens. The lemniscus fibres often course less deeply than (2) and give the stratum lemnisci. 

 The optic fibres proper approach the nucleus by way of the superior brachium, and are dispersed 

 directly among its cells; only a small proportion of them cross over to terminate in the nucleus 

 of the opposite side. They consist of two varieties: — (1) retinal fibres which arise in the gang- 

 lion-ceU layer of the retina and enter the superior brachium at its junction with the lateral root 

 of the optic tract, and (2) fibres from the visual area of the occipital lobe of the cerebral hemi- 

 sphere. Sometimes the optic fibres in their course within the nucleus of the superior coUiculus 

 form a more or less evident stratum near the stratum album profundum. This is known as the 

 stratum opticum (stratum album medium). The portion of the nucleus between this stratum 

 and the stratum zonale is called the stratum cinereum. 



The fibres entering the nucleus from the lateral lemniscus probably all represent auditory 

 connections. The stratum album profundum, composed of the lemniscus fibres and fibres 

 from cells of the nucleus, and the stratum opticum together, form the so-caUed 'optic-acoustic 

 reflex path' (fig. 662). 



The mesencephalo-spinal amd the spino-mesencephalic (spino-tectal) paths course together 

 ventro-lateral to the nuclei of the coUiculi. In the superior quadrigeminate bodies they course 

 in the dorsal edge of the median lemniscus, between the stratum opticum and stratum album 

 profundum. 



From the various studies that have been made it appears that the superior colliculus of the 

 corpora quadrigemina is merely the central reflex organ concerned in the control of the eye 

 muscles — eye muscle reflexes which result from retinal and cochlear stimulation, and from some 

 general body sensations by way of the spinal cord. Fibres from its nucleus to the visual area 

 of the occipital cortex have been claimed for certain mammals, but in man the superior colliculus 

 may be entirely destroyed without disturbance of the perception of light or color and fibres 

 arising from its nucleus to terminate in the cerebral cortex are denied. 



In the level of the anterior part of the superior colliculus the fibres which arise from the cells 

 of its nucleus and course ventrally in the stratum album profundum collect into a strong bundle. 

 This bundle passes ventral to the medial longitudinal fasciculus and, in the space between the 

 two red nuclei, it forms a dense decussation with the similar bundle from the opposite side. In 

 decussating the fibres turn in spray-hke curves downward and soon join the medial longitudinal 

 fasciculus. This is the 'fountain decussation' of Forel. It is said to be augmented by decus- 

 sating fibres from the two red nuclei. 



There is abundant evidence that fibres arising in the corpora quadrigemina descend into 

 the spinal cord. Various studies make it appear that at least part of these are fibres from the 

 fountain decussation, and that these course through the medulla oblongata in the ventral part 

 of the medial longitudinal fasciculus, and thence descend into the cord in the 'quadrigemino- 

 thalamus path' (lateral mesencephalo-spinal tract) (fig. 619). The medial longitudinal fasci- 

 culus is continuous with the ventral fasciculus proprius of the spinal cord and most of these 

 fibres arising in the superior quadrigeminate bodies retain their ventral position in the cord 

 as the sulco-marginal fasciculus of the opposite side. Their termination about those ventral 

 horn cells of the cervical cord which send fibres through the rami communicantes probably 

 establishes the pathway by which the superior quadrigeminate bodies are connected with the 

 cervical sympathetic ganglia, and by which may be explained the disturbances in pupillary 

 contraction induced by lesions of the lower cervical cord. 



The medial geniculate body and the medial root of the optic tract, which runs into the 

 former, probably have nothing to do with the functions of the optic apparatus. Both remain 

 intact after extirpation of the eyes. The medial root of the optic tract is apparently nothing 

 more than the beginning of the inferior cerebral (Gudden's) commissure, a bundle passing by 

 way of the optic tract, connecting the medial geniculate body of one side with that of the other 

 side, and prol^ably with tlie inferior colliculus. 



The medial longitudinal fasciculus (posterior longitudinal fasciculus), con- 

 tinuous into the ventral fasciculus proprius and the sulco-marginal fasciculus of 

 the spinal cord, extends throughout the rhombencephalon and mesencephalon, 

 and is represented in the hypothalamic region of the prosencephalon. Deserted 

 by the lemniscus at the inferior border of the pons, it maintains its closely medial 

 position and courses througliout in the immediate ventral margin of the central 

 grey substance of the medulla and floor of tlie fourth ventricle, and likewise 

 in the ventral margin of the central grey substance of the mesencephalon. 



