120 NERVOUS SYSTEM. 



various parts of the brain one to another. What is the function 

 of the gray matter is little known, but it is directly continuous 

 with that of the medulla, and probably, like it, active as a centre 

 of nervous force. The median line of the pons is marked by the 

 decussation of many nerve-fibres, and it is probable that the fibres 

 of the facial nerve arising in the floor of the fourth ventricle decus- 

 sate here. 



What peculiar paralyses are caused by lesions of the pons ? 



The so-called crossed paralysis may follow lesions in the lower 

 portion of the pons ; that is, paralysis of sensation and motion, 

 more or less complete, of the opposite side of the body, with paraly- 

 sis of the facial muscles of the same side as the lesion. 



Describe the crura cerebri. 



The crura are formed largely of fibres passing from the medulla, 

 through the pons Varolii, to the hemispheres of the cerebrum. They 

 divide so as to form two sets of fibres : the more superficial (crusta) 

 are mostly motor or efferent fibres which are continuous with the 

 pyramidal tracts in the cord ; while the deeper (tegmentum) layer 

 of fibres are afferent or sensory, and are derived largely from the 

 lateral and posterior tracts of the cord. Lying between these 

 bands of fibres is a mass of gray substance (locus niger) whose 

 function as a nerve-centre is not understood, though it has to do 

 with co-ordination of the muscles, and especially with regulation 

 of the muscles controlled by the motor oculi nerve. 



What paralyses follow lesions in the crura cerebri? 



Paralysis of the opposite side of the body, both of sensation and 

 motion, and of a degree of intensity depending upon the size of the 

 lesion, and, besides this, paralysis of the motor oculi nerve of the 

 same side as the lesion. There is a derangement of the ordination 

 of motions which follows lesions of this region beyond that which 

 belongs to the motor paralysis ; this is often shown in rotary move- 

 ments when the subject attempts to walk. It is inferred that there 

 are co-ordinating influences derived from the crura. 



What is the function of the corpora quadrigemina ? 



The corpora quadrigemina are the homologues of the optic lobes 

 in some of the lower animals, and may be regarded as important 

 centres for the visual and motor functions of the eyes. Not only 

 does blindness follow lesions of the corpora quadrigemina, but 

 there is often atrophy of them when the eyes are destroyed. 



