248 ANIMAL FORCES. [ii. 



out their whole extent (446), that_, like the heart, they move 

 actively on being irritated, when separated even for a lengthened 

 period from the body (357). See Haller's ' Physiology,' § 402, 

 and ' Opera Minora,' torn, i, pp. 384, 199, 390, &c. 



467. The co-operation of neither the cerebral forces nor the 

 mind is necessary to the natural functions of the oesophagus, 

 stomach, and intestines, excited as nerve-actions by external 

 impressions ; they can take place in animals deprived of both 

 (418). 



468. The oesophagus and gastro-intestinal canal are directly 

 excited by numerous external stimuli. Food of various kinds, 

 air, medicines, poisons, injuries, &c., excite contractions at the 

 point of irritation, which are propagated to their whole extent 

 as a result of the direct nerve-actions of the external impression 

 (446). Those excitants, which can excite an agreeable external 

 sensation, produce only natural movements in these viscera : 

 those which would excite pain, excite convulsive contractions, 

 as in muscles (compare § 452). 



469. In accordance with these views, it follows that the 

 entire process of digestion, considered as an animal function, 

 is the result of the nerve-actions of the external impressions 

 derived from the food in contact with the digestive organs, and 

 it may, therefore, take place quite independently either of 

 brain or mind. The movement of the chyle in the chyliferous 

 ducts belongs also to this class of functions. See Haller's 

 ' Opera Minora,' torn, i, pp. 378-9. 



470. The ordinary stimuli of the vermicular motions of the 

 alimentary canal, are not felt or propagated beyond its tissues. 

 We feel nothing of the food after it has passed beyond the 

 tongue : the violent irritants, which excite spasmodic action 

 and convulsive movements, are so little felt, that their presence 

 is only inferred from the rumbling of flatus, or from their 

 indirect nerve- actions, or from the external sensations excited 

 thereby in widely distant parts (212); as when w^orms in the 

 stomach or bowels excite convulsions, paralysis, pleurodynia, 

 frequent nausea and vomiting, &c. 



471. The functions of glandular secretion and excretion 

 are, for the most part, regulated by physical laws (159); but 

 since a gland is a tissue compounded of blood-vessels and 

 nerves, these functions must depend upon the influence of the 



