118 HAND-BOOK OF PHYSIOLOGY. 



single layer of branched nerve-cells, which give off a single unbranched 

 process downward, and numerous processes up into the external layer, some 

 of which become continuous with the scattered corpuscles. (3.) The 

 granular layer (g], consisting of immense numbers of corpuscles closely 

 resembling those of the nuclear layers of the retina. (4. ) Nerve fibre 

 layer (/). Bundles of nerve-fibres forming the white matter of the 

 cerebellum, which, from its branched appearance, has been named the 

 "arbor vitae." 



Functions. The physiology of the Cerebellum may be considered in 

 its relation to sensation, voluntary motion, and the instincts or higher 

 faculties of the mind. Its supposed functions, like those of every other 

 part of the nervous system, have been determined by physiological experi- 

 ment, by pathological observation, and by its comparative anatomy. 



(1.) It is itself insensible to irritation, and may be all cut away with- 

 out eliciting signs of pain (Longet). Its removal or disorganization by 

 disease is also generally unaccompanied by loss or disorder of sensibility; 

 animals from which it is removed can smell, see, hear, and feel pain, to 

 all appearance, as perfectly as before (Flourens; Magendie). Yet, if any 

 of its crura be touched, pain is indicated; and, if the restiform tracts of 

 the medulla oblongata be irritated, the most acute suffering appears to 

 be produced. So that, although the restiform tracts of the medulla 

 oblongata, which themselves appear so sensitive, enter the cerebellum, it 

 cannot be regarded as a principal organ of sensation. 



(2.) Co-ordination of Movements. In reference to motion, the experi- 

 ments of Longet and many others agree that no irritation of the cerebel- 

 lum produces movement of any kind. Eemarkable results, however, are 

 produced by removing parts of its substance. Flourens (whose experi- 

 ments have been confirmed by those of Bouillaud, Longet, and others) 

 extirpated the cerebellum in birds by successive layers. Feebleness and 

 want of harmony of muscular movements were the consequence of remov- 

 ing the superficial layers. When he reached the middle layers, the ani- 

 mals became restless without being convulsed; their movements were vio- 

 lent and irregular, but their sight and hearing were perfect. By the 

 time that the last portion of the organ was cut away, the animals had 

 entirely lost the powers of springing, flying, walking, standing, and pre- 

 serving their equilibrium. When an animal in this state was laid upon 

 its back, it could not recover its former posture, but it fluttered its wings, 

 and did not lie in a state of stupor; it saw the blow that threatened it, 

 and endeavored to avoid it. Volition and sensation, therefore, were not 

 lost, but merely the faculty of combining the actions of the muscles; and 

 the endeavors of the animal to maintain its balance were like those of a 

 drunken man. 



The experiments afforded the same results when repeated on all classes 

 of animals; and from them and the others before referred to, Flourens 



