GENERAL FUNCTIONS OF THE ENCEPHALON. 191 



obliged to keep her eyes constantly fixed on any thing (even her child) which 

 she held in her hands, as she could not continue the muscular effort when no 

 longer informed, by one sense or the other, that it was necessary. Now, the 

 only real difference between the case of the ordinary muscles and that of the 

 muscles of the eyeball, is (as Dr. Alison* has justly remarked) that the 

 guiding sensations are those received through the Retina in the latter case, 

 whilst in the former they are those of the muscles themselves. It may be 

 asked in what such consensual movements, as those of the Eye, differ from 

 those of a reflex character? The answer is, simply, that the former cannot be 

 effected without consciousness, and some mental condition supervening upon 

 it; whilst, in the latter, sensation has been shown not to be a necessary link. 

 The former may be as much involuntary as the latter, as is shown in the 

 effects of tickling, which could not be manifested in an unconscious individual. 

 Here a condition, very much resembling an emotion, is produced; and from 

 this, as from other emotions, various combined movements may result, with 

 which Volition has nothing to do. The same may probably be said of the 

 Instinctive actions of animals, which, as will presently appear, are probably 

 to be referred to the same category with the purely Emotional acts of Man : in 

 both, Sensation, and that usually of a special kirfd, is a necessary link. Further 

 it would appear that actions, which were originally of a completely voluntary 

 character, may come by habit to be performed within the shorter channel: thus, 

 a musician will play a difficult piece, whilst keeping up a conversation on 

 an entirely different subject; and here the muscular movements are guided, not 

 only by the sensations produced by their own contraction, but also by the anti- 

 cipation of the auditory sensations which will result from their operation. 

 The same may be said of the action of the muscles of Voice ( 412). 



;'" XVII. General Functions of the Encephalon. 



258. The portion of the Nervous Centres contained within the cranium, and 

 commonly designated collectively as the- Encephalon, may be regarded as con- 

 sisting of four principal divisions: 1, the Cerebral Hemispheres, which, in the 

 Mammalia, and especially in Man, constitute by far the largest portion of the 

 whole ; 2, the Cerebellum, the complete separation of which from the Cerebrum, 

 and its distinct connections with the Medulla Oblongata, mark it out as an organ 

 of peculiar character ; 3, the Tuber cula Quadrigemina and other Ganglionic 

 masses at the^Mfe of the brain, connected with the nerves of special sensation, 

 and analogous to the Olfactive, Optic and Auditory ganglia of the lower ani- 

 mals ; and 4, the Medulla Oblongata, or cranial prolongation of the Spinal 

 Cord, which is connected, at its upper end, with the Ganglia of specral sensa- 

 tion, with the Thalami optici, which may probably be regarded asfne corre- 

 sponding recipients of ordinary sensory impressions, and with the Corpa 

 Striata, through which the motor impulses are transmitted to it from the Hemi- 

 spheres. It has been already shown that this last organ is peculiarly connected 

 with the functions of Respiration and Deglutition ; and we shall next inquire 

 what special function can be attributed to the ganglionic enlargements at its 

 upper end. 



XVIIfc Functions of the Tubercula Quadrigemina, fyc. Emotional and 



Instinctive Actions. 



259. The degree in which animals high in the scale of organization can 

 perform the functions of life, without any other centre of action than the Gang- 



* Anatomical and Physiological Inferences from the Study of the Nerves of the Orbit, 

 in Trans, of Royal Society of Edin., vol. xv. 



