MA^'UAI. OF AGEICULTUIiE. 247 



ammonia, and nitric acid, animal bodies, on the other hand can 

 derive them from such complex organised compounds only as 

 are formed by plants out of the simpler elements. This prepares 

 us for the imjjortant fact, that all the actions of animal life con- 

 sist in the liberation of heat or force attendant upon the dis- 

 or<Tanisation of the organic compounds forming the tissues. Con- 

 sequently, ever}^ movement of the animal implies a consump- 

 tion or using up of materials in its frame. Muscular action is 

 the contraction and expansion of the delicate fibres compjosing 

 muscle structure in obedience to nervous stimulus ; and such 

 contraction is caused by a liberation of atoms or molecules, and 

 the resulting disorganisation or breaking np into simple com- 

 pounds of the proximate substances composing the muscle fibres. 

 Plants prepare their proximate constituents from the simple in- 

 organic compounds by sun heat and light agency ; whilst animals 

 derive their possible existence from the liberation of latent force 

 when these organised compounds are broken up. Plants also 

 absorb carbonic acid and give off oxygen ; animals inhale oxygen 

 and exhale carbonic acid. 



Nevertheless there are to be met with in the lowest scale of 

 development forms of animal life devoid of stomach, nervous 

 system, and independent locomotion ; and also vegeta,ble forms 

 endowed with some degre(3 of locomotion, and organs function- 

 ally resembling the stomach, and which do not obey the plant 

 laws of exhaling pure oxygen and subsistence upon inorganic 

 compounds. 



As in the vegetable, so in tlie animal kingdom is it with 

 regard to the fundamentally structural nature of the simple cell, 

 created froui a nucleus and the organic compound protoplasm by 

 that mysterious agency hitherto only known and defined as 

 " vital force." 



The functions treated of in animal physiology come under the 

 three heads of Nutrition, lieproduction, and Con-elation, which 

 last includes the consideration of the functions of sense and 

 motion, or those by which the total organism is brought into 

 relation with external nature. 



Leginning in order, we find in all the more highly-developed 

 classes an alimentary canal, into which is received food material, 

 undergoing those ])rocesses which render it tit for assimilation. 

 Then it is passed along the tortuous channel, where its nutritive 

 elements are absorbed and thence conveyed to the blood, and at 

 whose extremity the residuum of indigestible matter is excreted. 

 The solid food when receiveil into the mouth is broken up by the 

 teeth, and mingled with saliva during mastication ; and thus 

 rendered into a pulpy mass ea.sy to be swallowed, and prepared 

 for stomachic action. Pesides softening the food the .saliva exerts 

 certain chemical intluences upon it ; notably converting insoluble 



