22 THE NERVOUS SYSTEM 



separated by a fold of dura mater, termed the tentorium 

 cerebelli (p. 109). It consists of a narrow central portion, 

 known as the vermis, and two lateral hemispheres. The 

 cerebellum establishes connexions with the spinal medulla 

 and the medulla oblongata by means of the restiform bodies, 

 with the pons by means of the brachia pontis (middle peduncles], 

 and with the mid-brain and cerebrum by the brachia con- 

 junctiva (superior peduncles]. The term " cerebello-pontine 

 angle" is sometimes used to indicate the region where the 

 brachium pontis enters the substance of the cerebellum 

 (Fig. 12). 



The cerebellum exercises a controlling influence over 

 muscular tonus, and its cortex is intimately connected with 

 the cortex of the motor area of the cerebrum. The latter, 

 however, governs the muscles of the opposite side of the 

 body, whereas the cortex of the lateral cerebellar hemisphere 

 is related to the homo-lateral muscles. Cerebellar lesions are 

 accompanied by inco-ordination and loss of equilibrating 

 power and are typically characterised by a reeling, staggering 

 gait. In the case of the cerebellum, as in the case of the 

 brain, the symptoms are modified by the mode of onset of 

 the lesion, and they are not so distinctive in slow-growing 

 tumours as they are when the onset is more rapid. In the 

 latter case, the patient tends to fall towards the side of 

 the lesion, owing to the loss of tonus in the homo-lateral 

 muscles, but in slowly progressing cases the patient learns to 

 appreciate the tendency and often counteracts too strongly, 

 so that he falls (or deviates in walking) to the opposite side. 



Owing to the loss of tonus-control, more work is thrown on 

 the motor cortex of the cerebrum than it is able to perform 

 efficiently. As a result, intentional tremor may be well-marked 

 in cerebellar lesions. 



In unilateral irritative lesions of the cerebellar cortex, cere- 

 bellar fits may occur. They are characterised by tonic spasms, 

 most marked in the homo-lateral limbs. 



In lesions of the vermis, retraction of the head and arching 



