RELATION OF DIGESTION WITH SENSATION. 239 



In birds, the nervous system bears as great a propor- 

 tion to the bulk of the body, compared with other ani- 

 mals, as the respiratory organs. Hence, the acuteness 

 of their sensations and their general superiority in mental 

 activity strongly contrasts with the sluggishness of rep- 

 tiles and the intellectual stupidity of fishes. 



Digestion has also intimate relations with respiration. 

 Emily. This is very obvious ; for one great use of 

 respiration is, to separate those portions of the food re- 

 ceived into the blood, which are unfit for the purposes 

 of ihe animal economy, as well as to expose to the reno- 

 vating influence of the air, the really nutritive materials, 

 It is the office of digestion also, to prepare the materials 

 which are to supply the places of others that are taken 

 away, and carried out from the system by means of the 

 respiratory organs. There must of necessity, therefore, 

 be a uniform proportion of energy between respiration 

 and digestion. 



Dr. B. This is the reason why birds possess suck 

 strong powers of digestion, and require food at frequent 

 intervals. In reptiles, on the contrary, the digestive 

 powers are limited, and entire abstinence is easily borne 

 for very protracted periods. In their torpid state, they 

 lie for months without the slightest morsel of food. 



Emily. And the reason is very obvious ; nothing is 

 carried away from the system by respiration since this 

 function is hardly perceptible, and consequently nothing 

 is required from the digestive process. 



Dr. B. There is a very close relation between di- 

 gestion, and sensation and motion. In the inferior ani- 

 mals, the principal purpose of the latter, is to assist in 

 the performance of the former ; both are constructed with 

 a direct reference to the nature of the food. The car- 

 nivorous animal must not only have a set of digestive 

 organs fitted to act upon raw flesh, but his senses must 

 be sufficiently acute to distinguish his prey at a distance ; 

 his muscular power must be sufficient to enable him to 

 pursue and overcome it ; his claws must be sharp and 



