8 GENERAL PHYSIOLOGY, 



ble kingdom, light, heat, and doubtless electricity, constitute 

 the most indispensable excitants. This will explain why 

 grains of corn, found in the tombs of the pyramids, have 

 remained dormant during long periods of years without 

 showing any sign of life, and become awakened, or, in other 

 words, vegetate, when exposed to external excitants. The 

 conditions of the animal globules are none the less com- 

 plicated; again, a certain form of burning produces rapid 

 changes in the cells of human skin, in the epidermis. These 

 exciting causes may be physical, chemical, or even may origi- 

 nate in the interior of the organism (being vital), and the 

 principal of these interior (or vital) causes is most certainly 

 that of innervation, or the influence of the nervous system 

 upon the vital elements. Moreover, the actions of the vari- 

 ous excitants may succeed each other, and so form a circuit 

 of influences, alternating in character ; in this way the ele- 

 ments of the surfaces (epithelium, epidermis), excited by 

 external causes, excite in their turn by the mediation of sen- 

 sitive nerves the nerve cells, and by means of the motor- 

 nerves, convey the excitation to the muscles or to other 

 elements on the surfaces, as, for instance, the glandular epi- 

 thelium, and consequently we may have vital excitations pro- 

 duced by excitations which were at first only mechanical. 



Let us note, moreover, that, with certain globules, these 

 excitations may cause a special action : thus, the gastric juice 

 irritates the intestinal epithelium, and no other ; again, the 

 spermatic corpuscle is the sole excitant of the ovule, and 

 thus efficiently arouses its functional activity or its develop- 

 ment. 



Briefly, these excitants can act with varying degrees of 

 intensity. In the highest degree these may immediately 

 destroy the globule ; thus poisons act more especially upon 

 one or another group of globules, and thus cause their 

 destruction. 



It is difficult to explain the nature of the phenomena by 

 which an excitant acts on a cell. Sometimes this has been 

 compared with the so-called catalytic action, whose especial 

 characteristic consists of the fact, that the body neither gives 

 nor takes any thing from the excited body (phenomena of 



place seems to enjoy free-will, can only transmit or reflect irrita- 

 tions that are received from different sources. Those acts which 

 seem on superficial examination the result of nerve-spontaneity, 

 are really only reflex actions. 



