76 ESSENTIALS OF PHYSIOLOGY. 



Summary. The cerebellum receives afferent impulses (1) by way 

 of the vestibular nerve from the labyrinth of the ear, the end-organs 

 in which are affected by changes in the position of the head, and (2) by 

 the afferent nerves from the muscles, from the end-organs which are con- 

 nected with muscle sense. Both these sets of impulses are known as 

 proprio-ceptive to distinguish them from the extero-ceptive impulses 

 conveying information as to the environment of the body. The 

 cerebellum discharges efferent impulses which maintain muscle tone 

 during rest, and which co-ordinate muscular movements during activity. 

 These functions are especially of importance in the maintenance of 

 equilibrium. The absence of the cerebellum is therefore attended by 

 loss of tone and power in the muscles, as well as by a condition of 

 inco-ordination or ataxia. Cerebellar ataxia is thus associated with 

 muscular weakness, whereas spinal ataxia is associated with exaggerated 

 muscular movements. 



SECTION VII. 

 THE MID-BRAIN. 



The mid-brain consists of the cerebral peduncles on its ventral 

 aspect and the corpora quadrigemina on its dorsal aspect. The cerebral 

 aqueduct runs through its substance and connects the third with the 

 fourth ventricle. The cerebral peduncles contain the portions of the 

 motor and sensory tracts which intervene between the fore-brain and 

 the pons. The corpora quadrigemina are two pairs of prominences, 

 the superior and inferior colliculi, seen on the dorsal surface. 



If a coronal section is made through the mid-brain, it is seen that 

 the cerebral peduncle is divided into a dorsal and a ventral portion 

 by a layer of grey substance containing pigmented nerve cells and 

 known as the substantia nigra (fig. 22). The portion of the peduncle 

 ventral to the substantia nigra is called the pes (base), and it consists 

 entirely of nerve fibres running longitudinally. The dorsal portion 

 is known as the tegmentum. The fibres which form the middle three- 

 fifths of the pes are the pyramidal fibres, and extend from the cortex 

 of the cerebral hemisphere of the same side to become the pyramid of 

 the medulla and the pyramidal tracts of the spinal cord. Some of the 

 fibres, as has already been stated, cross in the hind- brain to the nuclei 

 of the cerebral motor nerves. The fibres of the medial fifth of the 

 base are the fronto-pontine fibres, and those of the lateral fifth are 

 the temporo-pontine fibres, forming connections between the frontal 

 and temporal cerebral lobes respectively and the nuclei pontis, and 

 thus with the contralateral lobe of the cerebellum. The fibres which 



