840 THE NERVOUS SYSTEM 



(nucleus of inferior colliculus) of the same side, and disappear among its cells; others cross the 

 mid-line to the quadrigeminate body of the opposite side. In crossing, some pass superficially 

 and thus contribute to the stratum zonale, while others pass either through the nucleus or below 

 it and cross beneath the floor of the median groove between the stratum zonale and the dorsal 

 surface of the central grey substance, forming there an evident decussation with similar fibres 

 crossing from the opposite side. Most of the fibres arising from the cells of the nucleus of the 

 inferior quadrigeminate body pass by way of the inferior brachium to the medial geniculate 

 body and the thalamus; some pass ventraUy to terminate in the nucleus of origin of the trochlear 

 nerve and some pass forward and laterally to terminate in the cortex of the superior gyrus of 

 the temporal lobe, the cortical area of hearing. Another portion of the lateral lemniscus passes 

 obliquely forward in company with the inferior brachium, and terminates in the medial gen- 

 iculate body. Thus a large portion of the lateral lemniscus, the inferior quadrigeminate bodies 

 with their brachia and the medial geniculate bodies are concerned with the sense of hearing. 

 The nucleus of the inferior quadrigeminate body receives fibres which arise in the cortex of the 

 superior temporal gyrus of chiefly the same side. 



Practically all the remainder of the lateral lemniscus terminates in the nucleus, or stratum 

 cinereum, of the superior quadrigeminate body of the same and opposite sides. They approach 

 the nucleus from below, and contribute to the well-marked band of fibres coursing on the dorso- 

 lateral margin of the central grey substance, and known as the ' optic-acovsiic reflex path' or 

 straivm lemnisci (fig. 662). 



The medial lemniscus arises in the medulla oblongata from the nuclei (of termination) of 

 the funiculus gracihs and funiculus cuneatus of the opposite side, and likewise from the nuclei 

 of termination of the sensory roots of the cranial nerves of the opposite side. It is, therefore, 

 a continuation of the central sensory pathway conveying the general bodily (including the 

 head) sensations into the prosencephalon. Coursing still more laterally than in the pons below, 

 it passes into the hypothalamic grey substance, in the lateral portion of which most of its fibres 

 terminate. By axones given off from the cells of the hypothalamic nucleus the impulses borne 

 thither by the lemniscus are conveyed by way of the internal capsule and corona radiata to the 

 gyri of the somajsthetic area of the cerebral cortex. 



The basis (pes) pedunculi comprises the great descending pathway from the 

 cerebral cortex, and thus is continuous with the internal capsule of the telen- 

 cephalon. 



The principal components of each basis pedunculi are as follows: — (1) The pyramidal 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 the somsesthetic area of the cerebral cortex, 

 chiefly the anterior central gyrus. These supply ' voluntary ' impulses to the motor nuclei of 

 the cranial nerves on the opposite side, form the pyramids of the medulla, and are distributed 

 to the ventral horn ceUs of the spinal cord of the opposite side. (2) The frontal pontile fibres, 

 which course in the mesial part of the peduncle from the cortex of the frontal lobe to their 

 termination in the grey substance of the pons. (3) The occipital and temporal pontile fibres, 

 which run in the ventral and lateral portion of the peduncle from their origin in the occipital 

 and temporal lobes to their termination in the grey substance of the pons. 



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

 that of the reticular formation below, and with that of the hypothalamic region 

 above. Its remarkable abundance begins at the superior border of the pons, and 

 it conforms to the crescentic inner contour of the cerebral peduncle, 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 suggesting its name. 



Its anatomical significance is not well understood. It is known that some fibres of the 

 medial lemniscus terminate about its cells instead of in the hypothalamus higher up, and Melius 

 has found in the monkey that a large portion of the pyramidal fibres arising in the thumb area 

 of the cerebral cortex are interrupted in the substantia nigra. It is probable that other fibres 

 of the peduncle 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 mesencephalon 

 as roundotl bundles. In the tegmentum, under the inferior colliculi, the two 

 brachia come togcither and undergo a sudden and complete decussation. Through 

 this dcicussation tlie filjres of the brachium of one side pass forward to terminate, 

 most of them, in the red nucleus [nucleus ruber] of the tegmentum of the opposite 

 side (fig. 589). Some fibres are said to pass the red nucleus and terminate in the 

 v(!ntrolat('ral part of the thalamus. 



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

 tegmentum under the superior fjuadrigeminate ])odies. At all levels they are 

 considerably mi,\(ul with the entering bundles of tlie brachia conjunctiva, and 

 th{y contain a ))igment wiiich in tlie fresh condition gives them a reddish colour, 

 suggesting their naine. 



They receive in addition descending fibres from the cerebral cortex (frontal operculum) 

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



