932 THE CENTRAL NERVOUS SYSTEM 



occipital cortex should light up there a visual sensation these are 

 questions to which we can as yet give no answer, and the answers to 

 some of which must for ever remain hidden from us. 



Functions of the Upper Part of the Central Stem and Basal Ganglia, 

 The function of the pons is sufficiently indicated by its name. The 

 grey matter so plentifully scattered, especially in its" ventral portion, 

 may exercise a not unimportant influence on the impulses that traverse 

 it. But on the whole its main office is to provide a bridge along which 

 impulses may travel between other portions of the nervous system. 

 We have already seen that many of its transverse fibres arising from 

 the cells of the pontine grey matter, and then crossing the middle line 

 to the opposite middle peduncle, are the cerebellar segments of com- 

 missural arcs connecting the cerebral with the opposite cerebellar 

 hemispheres. The cerebral segments of these arcs are the cortico- 

 pontine fibres originating in the prefrontal, temporal, and occipital 

 portions of the cerebral cortex, and passing through the corona radiata, 

 internal capsule, and crura cerebri, to end in the nuclei pontis. Many 

 fibres and collaterals of the pyramidal tract also terminate here. On 

 the dorsal aspect of the pons in the floor of the fourth ventricle are the 

 nuclei of origin (or reception) of the fifth, sixth, and seventh cranial 

 nerves. Various reflex centres are situated in this region e.g., that 

 for the closure of the eyelids, when the conjunctiva is stimulated. 



The posterior corpora quadrigemina and internal geniculate bodies are 

 connected with the cochlear division of the auditory nerves, and form 

 important stations on the auditory path to the cortex. 



The anterior corpora quadrigemina and the lateral corpora geniculata 

 are connected with the optic tracts. Their development is arrested 

 after extirpation of the eyeball in young animals, and they may there- 

 fore be assumed to be concerned in vision, although the size of their 

 homologues, the optic lobes or corpora bigeniina, in animals below the 

 rank of mammals (birds, reptiles, amphibians), does not seem to be 

 related to the development of the organs of sight. Proteus and the 

 Hag-fish, e.g., have large optic lobes, rudimentary eyes and optic tracts. 

 The optic nerve, the anterior corpus quadrigeminum, the nucleus of the 

 oculo-motor nerve in the wall of the Sylvian aqueduct, and the fibres 

 which it carries to the iris, form a reflex arc for the contraction of the 

 pupil to light, as represented in Fig. 370, p. 923. 



The functions of the optic thalami have not been fully defined either 

 by experiment or pathological observation, except in so far as they can 

 be deduced from then: connections. Lying as they do in the isthmus 

 of the brain, begirt by the great motor and sensory paths, it is to be 

 expected that lesions of the thalami should affect also the internal 

 capsule, and give rise to the symptoms of motor and sensory paralysis. 

 But it is questionable whether any definite defect of motor power or 

 common sensation has ever been unequivocally associated with a lesion 

 restricted to the thalami. The most constant features of the so-called 

 thalamic syndrome (or symptom-complex) are partial loss of sensibility, 

 especially to tactile impressions, and of the muscular sense on the 

 opposite side, with some degree of inco-ordination and disorder, though 

 little, if any, actual paralysis of voluntary movements. These phe- 

 nomena are accounted for by the extensive connections of the thalami. 

 Each of the thalamic nuclei is linked with a definite cortical region in 

 such a way that destruction of the cortical area in young animals or 

 human beings leads to degeneration of the corresponding nucleus. 

 Some of the fibres connecting the cortex (and the corpus striatum) 

 with the thalamus end in the thalamic grey matter, and are therefore 

 efferent with respect to the cortex (corticof ugal) . It is, however, the 



