710 A TEXTBOOK OF PHYSIOLOGY 



limb run fibres connected with movements of the head and eye.s; in 

 the genu with the movements -of the tongue and mouth; in the 

 anterior two-thirds of the posterior limb the fibres which go to form 

 the pyramidal tract in the following order from before backwards: 

 shoulder, elbow, wrist, fingers, trunk, knee, toes (Fig. 433). 



In the posterior part of the posterior limb of the capsule run 

 fibres from the occipital cortex to the pons and cerebellum (the 

 occipito-pontine fibres), and in front of and behind these other fibres, 

 known respectively as the auditory and optic radiations, connected 

 with the sensory cortical areas for hearing and sight. The fibres of 

 the auditory radiation pass from the inferior corpus quadrigeminum 

 and internal geniculate body to the temporo-sphenoidal lobe of the 

 brain; those of the occipital radiation from the optic thalamus and 

 external geniculate body to the occipital cortex. 



The Optic Thalamus consists of a mass of white and grey matter 

 situated to the inner side and below the floor of the third ventricle 

 of the brain. It receives fibres from (1) the cuneate and gracile 

 nuclei of the opposite side by the mesial fillet; (2) the opposite dentate 

 nuclei of the cerebellum by the superior cerebellar peduncles; (3) the 

 retina by the optic tract; (4) the cortex of the same side. Its cells . 

 give rise to fibres which pass (1) to the cortex as the last link of the 

 great sensory chain to the motor area of the cerebral cortex; (2) to 

 the part of the cortex concerned in vision; (3) to fibres which run in 

 the rubro-spinal tract to the cord. 



The Function of the Thalamus. The nature of the function of 

 the thalamus has been adduced chiefly as the result of the clinical 

 evidence obtained from diseases of this part of the brain. In a lesion 

 of the thalamus there is persistent loss of superficial sensation, 

 touch, pain, and temperature of half the body. There is also 

 marked loss of deep sensation and acute pains on the affected side. 

 There may be more or less complete want of knowledge of structure 

 and form in the sense of touch (astereognosis). There is but little 

 affection of the motor system. There may be slight ataxy on the 

 affected side, or a partial hemiplegia, which quickly passes off. There 

 may be " choreic '' and " athetotic " (twirling) movements of the 

 affected side. These, however, are not due to the thalamus; only 

 sensory affections are caused by thalamic lesions. 



From the clinical evidence which has accumulated, it seems fairly 

 certain that the impulses which pass up the posterior columns of the 

 cord to the gracile and cuneate nuclei are so rearranged during their 

 passage up the mesial fillet to the thalamus that the fibres connect e; I 

 with each kind of cutaneous sensation arborize round definite groups 

 of cells within the thalamus, whence other axons arising in these 

 groups convey the impulses to cortical areas. The thalamus is there- 

 fore a great sensory centre. It responds to all stimuli capable of 

 evoking either pleasure and pain (feeling tone) or consciousness of a 

 change in state. The feeling tone of somatic or visceral sensation is 

 the product of thalamic activity, and the fact that a sensation is 

 devoid of feeling tone shows that the impulses which underlie its 





