THE CENTRAL NERVOUS SYSTEM 829 



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 

 therefore be assumed to be concerned in vision, although the size 

 of their homologues, the optic lobes or corpora bigemina, 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 \vhich it carries to the iris, form a reflex arc for the 

 contraction of the pupil to light, as represented in Fig. 342, p. 819. 



The functions of the optic thalami have not been fully defined 

 either by experiment or pathological observation, except so far as 

 they can be deduced from their 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 unequivo- 

 cally 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 impres- 

 sions, 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 phenomena 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 afferent 

 paths to the cortex with which the thalami are specially related 

 as centres of relay. The fibres of the upper fillet carrying afferent 

 impulses up from the opposite posterior column of the cord to the 

 cerebrum end in the grey matter of the thalamus, as does the 

 central path of the afferent fibres of the opposite fifth nerve. The 

 posterior portion of the thalamus, or pulvinar, forms part of the 

 central visual apparatus ; for (a) it is found to be undeveloped in 

 animals from which the eyeballs have been removed soon after 

 birth ; (6) a portion of the optic tract is certainly connected with it ; 

 (c) in some cases of atrophy of the occipital cortex, which, as we 

 shall see, is undoubtedly a central area for visual sensations, atrophy 

 of the pulvinar has also been noticed ; (d) a lesion of the pulvinar 

 may give rise to hemianopia (p. 819). 



Haemorrhage into the caudate or lenticular nucleus of the corpus 

 striatum often causes hemiplegia, but this is frequently due to implica- 

 tion of the internal capsule. It is said, however, that lesions presumably 

 confined to the lenticular nucleus cause paralysis or paresis of the 

 limbs or face, which is less severe than that produced by lesions in 

 the internal capsule. Experimental lesions in dogs and rabbits 

 are stated to be followed by disturbances of the heat-regulating 

 mechanism and rise of temperature. 



Certain structures belonging to the primary fore-brain which have 

 now lost some or all of their functional importance, may neverthe- 



