82 i 



is, therefore, a continuation of the central sensory pathway convoying the general 

 bodily sensations into the prosencephalon. In passing the superior (|uadrigeniinale 

 body it contributes fibres to its nucleus. The remaining part , coursing still inure later- 

 ally than below, pas.-cs into the hypoi halainic grey substance, in the lateral portion 

 of which most of its tibres terminate, r'roin the t halainic region the impulses borne 

 thither by the lemnisrns are conveyed by way of the internal capsule and corona 

 radiata to I lie gyri of the sonuesthel ic area of the cerebral cortex. 



The basis (pes) pedunculi comprises the great descending j)athway from the cere- 

 bral cortex, and thus is continuous with the internal capsule of the telencephalon. 

 The principal components of each basis pedunculi are as follows: (1) The pi/ru- 

 iniilnl fibres, which occupy the middle portion of the peduncle and comprise three- 

 fifths of its bulk, and which are outgrowths of the giant pyramidal cells of thesonia-s- 

 t hctic area of the cerebral cortex. These supply 'voluntary' impulses to the motor 

 nuclei of the cranial nerves, form the pyramid:- of the medulla, and are distributed 

 to the vent ral horn cells of the spinal cord. (2) The frontal pantile /Hires, which course 

 in t lie mesial part of the peduncle from the cortex of the frontal lobe to their termina- 

 tion in the grey substance of the pons. (3) The temporal pantile fibres, which run 

 in the lateral portion of the peduncle from their origin in the temporal lobe to their 

 termination in the grey substance of the pons. 



The substantia nigra is continuous with the grey substance of the pons and of 

 the reticular formation below, and with that of the hypot halainic region above. Its 

 remarkable abundance begins at the superior border of the pons, and it conforms to 

 the crescent ic inner contour of the basis pcduticnli, sending numerous processes 

 which occupy the inter-fascicular spaces of the latter. It contains numerous deeply 

 pigmented nerve-cells, which in the fresh specimen give the appearance sutrireMJng 

 itfl name. Its anatomical significance is not well understood. It is known that 

 some fibres of the medial leninisctis terminate about its cells instead of in the 

 hypot hahimus higher up, and Mellus has found in the monkey that a large portion 

 of t he pyramidal fibres arising in the thumb area of t he cerebral cortex are interrupted 

 in the substantia nigra. It is probable that other fibres of the basis pedunculi also 

 terminate here. 



The brachia conjunctiva or superior cerebellar peduncles, in passing from their 

 origin in the dentate nuclei, lose their flattened form and enter the mesenccphalon as 

 rounded bundles. In the tegmentilm, under the inferior colliculi, the two brachia 

 come together and undergo a sudden and complete decussation. Through this 

 decussatipn the fibres of the brachium of one side pass forwards to terminate, most 

 of them, in the red nucleus of the tegmentum (nucleus rul>er) of the opposite side 

 (fig. ">89). 



The red nuclei are two large, globular masses of nerve-cells situated in the 

 tegmentum under the superior quadrigeminate bodies. At all levels they are con- 

 siderably mixed with the entering bundles of the brachia conjunctiva, and' they con- 

 tain a pigment which in the fresh condition gives them a reddish colour, suggesting 

 their name. They receive in addition descending fibres from the cerebral cortex 

 and from the nuclei of the corpus striatum. From the cells of each red nucleus 

 arise fibres which pa.>s -(1) into the t hahimus and to the telencephalon (prosenceph- 

 alic continuation of the cerebellar path), and (2) fibres which descend into the 

 spinal cord, the 'rubro-spinal tract.' in the intermediate fasciculus (fig. 571). The 

 latter cross from the red nucleus of the opposite side and descend in the tegmentum. 

 The red nuclei are also in relation with the jnxcieulitx retrofteXUS of .Mevnert . which 

 belongs to the inter-brain. 



The superior quadrigeminate bodies (superior colliculi) are phylogenetically 

 more important than the inferior. In certain of the lower vertebrates they are 

 enormously developed and in most of the mammals they are rclativelv larger arid ap- 

 pear moie complicated in structure than in man. They are concerned almost wholly 

 with the visual apparatus. 



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

 the inferior. It is capped by a strong afratorn :>in<il< (fig. Ill I), 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 (lie eye is 

 followed by very slight degeneration of the stratum /onale. it is probable) hat it is com- 

 posed of other than retinal fibres possibly fibres from the occipital cortex anil fibres 

 arising within the nucleus itself. The nucleus is separated from the central grey 



