CHAP. XI.] FUNCTIONS OF THE ENCEPHALON. 345 



with excitation of this centre, either direct, or propagated to it 

 from some sentient surface. 



This portion of the encephalon is also the centre of action in the 

 movements of deglutition, through fibres of the glosso-pharyngeal 

 and vagi nerves. A morbid state of it occasions difficulty, or even 

 paralysis, of deglutition. Animals deprived of the cerebral hemi- 

 spheres and cerebellum will preserve the power of swallowing food 

 introduced within the grasp of the fauces, so long as the medulla 

 oblongata continues uninjured. In foetuses born without cerebral 

 hemispheres, those actions are present which depend on the spinal 

 cord and medulla oblongata ; all the movements of respiration and 

 deglutition are performed as well as in the perfect foetus. Mr. 

 Grainger' s experiments shew that puppies deprived of the hemi- 

 spheres of the brain can perform the movements of suction with con- 

 siderable vigour, when the finger is introduced into the mouth ; * 

 and the remarkable fact of the adhesion of the foetus of the kangaroo 

 to the nipple within the pouch, no less than its respiratory move- 

 ments, must, as this author remarks, be regarded as a most 

 interesting display of the physical power of the medulla oblongata, 

 while the rest of the brain is as yet undeveloped. 



The actions of respiration and deglutition are, to a great extent, 

 of the physical kind, being excited by impressions propagated from 

 the periphery. In those of respiration, the ordinary exciting cause is 

 probably, as Dr. Hall supposes, due to the chemical changes in the 

 respired air which are effected in the lungs. These movements 

 may be, to a certain extent, controlled by the will ; but every one 

 is conscious, from his own sensations, that after a time the physical 

 stimulus is capable of conquering the restraining influence of the 

 mind ; a striking example of a mental stimulus giving way to a 

 physical one ; and illustrative, as we think, of the doctrine that the 

 same fibres are affected by both stimuli. The excitation of the 

 medulla oblongata in respiration does not, however, depend solely 

 upon the pulmonary nerves. Those of the skin are capable of 

 exciting it, either directly as the fifth pair, or through the spinal 

 cord, as it is proved by the inspirations which are instantly excited 

 by suddenly dashing water on the face or trunk. 



In deglutition, the exciting cause is the stimulus of contact 

 applied to the mucous membrane of the fauces. So highly sen- 

 sitive is the mucous membrane in this situation, that the slightest 

 touch of it with a feather is sufficient to produce contraction of the 



* Loc. cit. pp. 80, 81. 



