ON THE EXTERNAL SENSES OF ANIMALS. 159 



rials in particular parts of it. Thus, opium and alkohol, the juice of aconite, 

 and essential oil of laurel or bitter almonds, produce little pr no effect upon 

 the absorbents of the skin, but a very considerable effect upon the coating of 

 the stomach. Jn like manner, carbonic acid gas invigorates rather than 

 injures, when applied to the absorbents of the stomach, but instantly destroys 

 life when applied to those of the lungs; while the aroma of the toxicaria 

 Macasariensis, or Boa upas, of which we have heard so much of late years, 

 proves equally a poison, whether received by the skin, the stomach, or 

 the lungs. 



So, also, substances that are poisonous to one tribe of animals are medi- 

 cinal to a second, and even highly nutritive to a third. Thus, swine are poi- 

 soned by pepper-seeds, which to man are a serviceable and grateful spice ; 

 while henbane-roots, which destroy mankind, prove a wholesome diet to 

 swine. In like manner, aloes, which to our own kind is a useful medicine, 

 is a rank venom to dogs and foxes ; and the horse, which is poisoned by the 

 phellandrum aquaticum, or water-hemlock, and corrosive sublimate, will 

 take a drachm of arsenic daily, and improve hereby both in his coat and 

 condition. 



It has already appeared, that the secernent vessels of any part of the sys- 

 tem, in order to accomplish a beneficial purpose, as, for example, that of re- 

 storing a destroyed or injured portion of an organ, may change their action, 

 and secrete a material of a new nature and character. An equal change is 

 not un frequently produced under a morbid habit, and the secretion will then 

 be of a deleterious instead of being of a healthy and sanative kind. And 

 hence, under the influence of definite causes, the origin of such mischiev- 

 ous and fatal secretions, in some instances thrown forth generally, and in 

 others only from particular organs, as the matter of srnall-pox, measles, putrid 

 fevers of various kinds, cancer, and hydrophobia, or the poisonous saliva of 

 mad dogs. 



But the field opens before us to an unbounded extent, and we should lose 

 ourselves in the subject if we were to proceed much farther. It is obvious, 

 that in organic, as in inorganic nature, every thing is accurately arranged 

 upon a principle of mutual adaptation, and regulated by an harrhonious anta- 

 gonism, a system of opposite yet accordant powers, that balance each other 

 with most marvellous nicety; that increase and diminution, L'fe and death, 

 proceed with equal pace ; that foods are poisons, and poisons foods ; and, 

 finally, that there is good enough in the world, if rightly improved, to make 

 us happy in our respective stations so long as they are allotted to us, and evil 

 enough to wean us from them by the time the grant of life is usually recalled. 



LECTURE XV. 



ON THE EXTERNAL SENSES OF ANIMALS. 



THE subject of study for the present lecture is the organs of external sense 

 in animals : their origin, structure, position, and powers ; and the diversities 

 they exhibit in different kinds and species. 



The external senses vary in their number : in all the more perfect animals 

 they are five ; and consist in the faculties of sight, smell, hearing, taste, and 

 touch. 



It is by these conveyances that the mind or sensory receives a knowledge 

 of whatever is passing within or without the system; and the knowledge it 

 thus gets possession of is called perception. 



The different kinds of perception, therefore, are as numerous as the different 

 channels through which they are received, and they produce an effect upon 



