8 INTRODUCTORY LECTURE. 



Permit me to give another instance of the necessity of studying 

 the whole organisation and relations of an animal in order to learn 

 the physiology of the modification of one of its organs. 



In tracing the progressive complications of the stomach, we at length 

 meet with it under that very singular condition which we term a 

 gizzard ; in which the cavity is reduced to a mere fissure, by the accu- 

 mulation of muscular fibres in its walls, and by a thick and callous 

 lining of dense horny matter. The physiologists who viewed this 

 modification of a stomach, without reference to the rest of the or- 

 ganisation of the bird, and who contented themselves by experimenting 

 upon the compressive and triturating force of the gizzard, were led to 

 conclude that digestion was mainly a mechanical process. They were 

 here misled by Comparative Anatomy ; but it was by its abuse. 



Graminivorous and granivorous birds — those species whose 

 food demands the most complete comminution — have that mecha- 

 nical process performed, it is true, exclusively by the gizzard ; but 

 near this triturating stomach we find another cavity as exclusively 

 secretory in its functions, and which we know, by experiment, to fur- 

 nish a powerful solvent in great quantities to act upon the comminuted 

 food. But why the comminuting machinery should be transferred 

 to the abdominal cavity in the bird requires for its explanation a 

 review of the general structure, habits and sphere of existence, of this 

 particular form of animal. 



The most prominent quality in the bird is its power of flight — to 

 lighten the extremities and accumulate the weight at the centre of 

 gravity favour this power : it is especially requisite that the head, 

 which is supported on a long and flexible neck, should be as light as 

 possible. To this end the jaws, instead of supporting dense and heavy 

 teeth, are wholly edentulous, and are sheathed with light horn ; they 

 are simply prehensile, not masticatory, organs ; and the muscular 

 masses, subserving mastication, are consequently uncalled for. The 

 compensation is admirably adjusted in harmony with the exigencies 

 of the bird : pebbles are swallowed to serve as teeth ; are collected in 

 the gizzard, near the centre of gravity, of the whole body, at which 

 point the muscular mass required to operate upon them, and, by their 

 means, to crush the grain, is likewise concentrated. Thus the teeth, and 

 masticatory muscles are removed from the head, and concentrated in 

 the stomach, at the centre of gravity of the bird ; and the peculiarities 

 of its stomach are thus found, by a general survey of the organisation 

 and habits of the animal, to relate to the acquisition of certain me- 

 chanical advantages in the disposition of the weight of the body, so as 

 to favour the act of flight. 



I might easily multiply such instances, but I should thus only 



