192 FUNCTIONS OF THE NERVOUS SYSTEM. 



lia of Special sense, the Medulla Oblongata, and the Cerebellum, appears extra- 

 ordinary to those who are accustomed to regard the Cerebral Hemispheres as 

 the centre of all energy. From the experiments of Flourens, Hertwig, Ma- 

 gendie, and others, it appears that not only Reptiles, but Birds and Mammalia, 

 may survive for many weeks or months (if their physical wants be duly sup- 

 plied) after the removal of the whole Cerebrum. It is difficult to substantiate 

 the existence in them of actual sensation ; but their movements appear to be 

 of a higher kind than those resulting from mere Reflex action. One of the 

 most remarkable phenomena in such a being, is the power of maintaining its 

 equilibrium, which could scarcely exist without consciousness. If it be laid 

 upon the back, it rises again ; if pushed, it walks. If a Bird thus mutilated be 

 thrown into the air, it flies ; if a Frog be touched, it leaps. Such a being, 

 when violently aroused, has all the manner of an animal waking -from sleep ; 

 and it manifests just about the same degree of consciousness with a sleeping 

 Man, whose torpor is not too profound to prevent his suffering from an uneasy 

 position, and who moves himself to amend it. The negative results of experi- 

 ments of this kind are much more satisfactory than the positive ; that is to 

 say, if we are able to substantiate the performance of a particular function, 

 after the removal of a certain organ, we may be sure that the function is not 

 dependent on that organ. But the converse does not hold good ; for it fre- 

 quently happens that, when such violent operations are practised on the ner- 

 vous centres, they occasion an amount of general disturbance, which suspends 

 or modifies functions that have no immediate connection with the organ in 

 question ; so that we cannot safely attribute the alteration in them to the loss 

 of it. For example, Hertwig found that, upon removing the upper part of the 

 hemispheres in a pigeon, the powers of sight and hearing appeared to be 

 destroyed, and the animal sat in one spot, as if asleep ; but, being fed during 

 a fortnight, the sensibility returned, and the bird lived for three months. 



260. Among the ganglia of special sensation, the functions of the Optic 

 Lobes, or Corpora Quadrigemina, have been chiefly examined. The researches 

 of Flourens and Hertwig have shown, that their connection with the visual 

 function, which might be inferred from their anatomical relations, is substan- 

 tiated by experiment. The partial loss of the ganglion on one side produces 

 partial loss of power and temporary blindness on the opposite side of the body, 

 without necessarily destroying the mobility of the pupil ; but the removal of a 

 larger portion, or complete extirpation of it, occasions permanent blindness and 

 immobility of the pupil, with temporary muscular weakness, on the opposite 

 sides. This temporary disorder of the muscular system sometimes manifests 

 itself in a tendency to move on the axis, as if the animal were giddy. No 

 disturbance of consciousness appears to be produced ; and Hertwig states that 

 he never witnessed the convulsions which Flourens mentions as a consequence 

 of the operation, and which were probably occasioned by his incision having 

 been carried too deeply. We shall now inquire what inferences may be drawn 

 from comparative observations, in regard to the more general function of these 

 organs. 



261. That a large number of the actions of the lower animals are immedi- 

 ately prompted by Sensations, without the intervention of reasoning processes, 

 is universally admitted ; and' to these actions the term Instinctive is ordinarily 

 given. They appear to result from the direct operation of a mental condition, 

 analogous to that which exists in Man, when the emotions, passions, or pro- 

 pensities are so strongly excited, as to act at once on the body without the 

 intervention of the Will. In the purely reflex movements, it has been shown 

 that sensation is not a necessary link. On the other hand, in Voluntary acts, 

 neither sensations nor emotions directly affect the body, but only serve to stimu- 

 late the reasoning j^rocesses, and to supply motives to the judgment ; and the 



