8 22 THE NERVOUS SYSTEM 



substance by a well-marked band of fibres, the stratum album profundum. This con- 

 tains fibres from two sources: (1) fibres from the lateral and medial lemnisci, 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 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 ganglion- 

 cell 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 hemisphere. Sometimes the optic fibres in their course within the nucleus 

 of the superior colliculus form a more or less evident stratum near the stratum alburn 

 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-called 'optic-acoustic reflex path' (fig. 611). 



From the various studies that have been made it appears that the superior col- 

 liculus 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 coch- 

 lear stimulation. 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 (if the perception of light or color. 



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-like 

 curves downwards and soon join the medial longitudinal fasciculus. This is the 

 'fountain decussation.' (Forel.) 



There is abundant evidence that fibres arising in the corpora quadrigemina de- 

 scend 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, probably no further than the cervical region, in the ' quadrigemino- 

 thalamus path' (fig. 571) . The termination of these crossed fibres 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, and 

 indirectly the retina, 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 Guddcn's commissure, a bundle 

 passing by way of the optic tract, connect ing the medial geniculate body of one side 

 with that of the other side. 



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

 tinuous with 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 throughout in the immediate ventral margin of the central grey sub- 

 stance of the central canal and floor of the fourth ventricle, and likewise in the 

 ventral margin of the central grey substance of the mesencephalon. The two fasci- 

 culi constitute the principal association pathways of the brain-stem, and, true to 

 their nature as such, they are among the first of its pathways to acquire medulla- 

 tion. In the mesencephalon they become two of its most conspicuous tracts, and 



