CHAP. XXIV.] PRINCIPAL PARTS OF THE BRAIN. 499 



But the effects of the foregoing sources of uncertainty are pro- 

 bably increased by what we shall find to be the well-grounded con- 

 sideration, that the Cerebellum, whatever may be the precise 

 nature of its functions, does not commonly act alone, but to a very 

 considerable extent in conjunction with the Cerebrum in the per- 

 formance of certain functions common to both. Thus it seems 

 not at all unlikely that in cases of injury or disease of the Cere- 

 bellum there may be some compensatory increased action of the 

 Cerebrum especially where the disease has lasted long or has 

 commenced at an early age, as in the case of atrophy of this 

 organ in -the girl examined by Combette, and whose case is re- 

 corded by Cruveilhier. Lastly, another cause of difficulty, tending 

 to complicate the interpretation of the results of diseaseof the 

 Cerebellum, may arise from the possibility that, in the case of uni- 

 lateral lesions, the sound half of the organ may be capable of 

 taking on and discharging after a fashion perhaps with a mere 

 difference in degree the functions of the disabled part (see p. 509, 

 note). 



In the face of all these difficulties of interpretation it 

 may be well to turn back and look at the problem as to 

 the functions of the Cerebellum by the light of general 

 principles, aided by any additional illumination which we 

 may be capable of obtaining from our modern knowledge 

 (so far as it goes), as to the precise anatomical connec- 

 tions of the organ with different parts of the Cerebrum 

 and with different tracts of the Spinal Cord. 



The Cerebro- Spinal System of Vertebrates contains a series of 

 ' sensory ' and 'motor' centres throughout the whole length of the 

 Spinal Cord and Medulla, each of which, whilst capable of per- 

 forming independent functions, is also in subordinate relation with 

 other higher Nerve Centres. 



Something similar obtains among Worms and Arthropods. 



But the Brain in all Vertebrates differs from that of Invertebrates, 

 in the fact that it possesses two double morphologically distinct 

 parts, unrepresented among the latter, or, at least, unrepresented 

 by similarly separable parts. These are the Cerebral Lobes and 

 the Cerebellum. Making their appearance as comparatively small 

 segments in Fishes, their relative size and development increases 



K K 2 



