706 A TEXTBOOK OF PHYSIOLOGY 



The cerebellum is therefore to be regarded as the chief centre for 

 the muscular co-ordination and adjustment necessary for the equilibra- 

 tion of the body. It receives its chief impressions from the proprio- 

 ceptive mechanisms of the head and body, the receptor mechanisms, 

 the semicircular canals, and the muscles, joints, and tendons, and also 

 from the retinae. Normally, these are in accord with the impulses 

 from the eye. If, however, they are not, then giddiness results, 

 as illustrated by the children's game already mentioned, in which the 

 subject, after walking round several times with the forehead on a 

 poker, stands upright and attempts to walk straight. The resultant 

 loss of balance is due to the conflict between the semicircular canals, 

 which give a sense of rotation in the opposite direction, and the eyes, 

 which afford no evidence of any such rotation. The giddiness experi- 

 enced in looking down from great heights is also well known. The 

 importance of the eyes is also seen in the ataxia which results from 

 lesions of the posterior columns of the cord affecting the kinsesthetic 

 mechanisms of the limbs. A patient suffering from such ataxia 

 balances well so long as his eyes are open. If these be closed the 

 power to equilibrate ceases. Should such a person close his eyes 

 while bending over a basin of water to wash his face, he will fall 

 forward into the basin. Such an incident is in some cases the first 

 sign of the spinal lesions. 



The cerebellum effects its control over the muscles concerned 

 in equilibration through the cerebrum of the opposite side, and 

 through the cranial nuclei and anterior horn cells of the cord. 



SECTION III 

 THE MESENCEPHALON, OR MID-BRAIN 



THE mid -brain may be looked upon as being chiefly made up of 

 four peduncles the two cerebral and the two superior cerebellar 

 peduncles. Superimposed upon the two latter are the anterior and 

 posterior quadrigeminal bodies, two on either side. Through the 

 mid-brain runs the aqueduct of Sylvius, connecting the fourth ventricle 

 to the third ventricle of the great brain. 



In section, a black pigmented zone on either side the substantia 

 nigra divides off an anterior part, or crusta, from a posterior part, 

 or tegmentum. The crusta consists principally of efferent fibres 

 from the cerebral cortex. In the centre are situated the pyramidal 

 fibres, on either side the fibres passing from the cortex to the nuclei 

 pout is and cerebellum the fronto-pontine and occipito-pontine 

 fibres. 



In the tegmentum run the afferent tracts, the mesial fillet to the 

 optic thalamus, the lateral fillet to the inferior corpora quadrigemina, 

 and the fibres which pass to and from the cerebellum by means of 

 the superior peduncles. In it are also situated masses of grey matter 

 (1) the red nuclei; (2) the nuclei of the fourth and third cranial 

 nerves. 



