30 THE NERVOUS SYSTEM 



a similar elevation on its inferior surface. This elevation is 

 termed the lateral geniculate body, and both it and the pulvinar 

 receive afferent fibres from the optic tract (Fig. 17). 



The superior surface of the thalamus has been mentioned 

 (p. 23) in connexion with the floor of the lateral ventricle, 

 and the medial surface in connexion with the lateral wall of 

 the third ventricle (p. 28). Its lateral surface (Fig. 18) is 

 in contact with the fibres of the internal capsule (p. 32). 

 Owing to this latter relationship, the functions of the thalamus 

 are extremely difficult to determine, as lesions are rarely 

 restricted to the thalamus itself, and the possible involvement 

 of the internal capsule cannot be excluded. 



Lesions in the posterior part of the thalamus usually affect 

 the pulvinar and the lateral geniculate body, and consequently 

 produce homonymous hemianopia (p. 52). In addition, there 

 is usually some degree of motor paralysis and of hemi- 

 ansesthesia. Loss of deep sensibility is frequently co-existent 

 with thalamic lesions and it is often accompanied by astereo- 

 gnosis and loss of muscle and joint sense. How far these 

 symptoms are due to involvement of the internal capsule, it is 

 at present impossible to determine. 



2. The Caudate Nucleus (p. 23) and (3) the Lentiform 

 Nucleus will be referred to in the description of the internal 

 capsule (vide infra]. 



The precise functions of these nuclei are not yet understood. 

 Bilateral lesions, e.g. progressive softening, of the lentiform 

 nuclei cause difficulty in articulation and tremors and spasticity 

 in the muscles of the trunk and lower limbs. Cases of this 

 kind have been recorded in which careful examination has 

 failed to detect any affection of the internal capsule. 



SECTIONS THROUGH THE BRAIN 



In Horizontal Transverse Sections made through the 

 cerebral hemisphere at the level of the interventricular 

 foramen, the island (of Reil) forms a conspicuous landmark. 



