90 GENERAL PHYSIOLOGY. 



adult, unless they are called forth by the action of various agents 

 named stimuli. Such stimuli, from their operation being peculiar and 

 essential to living bodies, are sometimes entitled "vital" stimuli; but 

 that term is more appropriately limited to such stimuli as actually 

 originate in living organisms. Speaking generally, the stimuli to life 

 are some external and some internal. The external stimuli are either 

 mechanical, physical, or chemical, such as pressure, friction, pricking, 

 or cutting, heat or cold, electricity, light, and various chemical agents. 

 Such stimuli, with the exception of light, if applied to a contractile or 

 muscular tissue, cause it to contract; if applied to the nervous tissues, 

 they rouse their excitability, bring into operation the conductility of 

 the nerve-fibres, and, through them, excite the special sensibilities of 

 different nerve-cells, or, indirectly, induce muscular contractility. 

 In the former case, one result only is produced, whatever be the 

 nature of the stimulus, viz., contraction or motion; in the latter case, 

 the effects differ according to the nature of the external stimulus, or 

 the character of the nervous excitability of particular nerves or ner- 

 vous centres, i. ., to their susceptibility to react in a peculiar manner 

 in relation to those stimuli. Thus pressure produces the sensation of 

 touch; changes of temperature the sensations of heat and cold; rapid 

 mechanical vibrations, hearing ; chemical actions excite taste and 

 smell, and luminous vibrations produce sight. The internal stimuli 

 are partly of the same nature as the external stimuli, as e. g., the 

 stimulus of food, which is partly mechanical and partly chemical, and 

 which may produce both motion in contractile tissues and sensation 

 in sensitive tissues. Of the same nature is the internal chemical 

 stimulus of the blood, chiefly due to the oxygen which it absorbs from 

 the air; and, lastly, to this category, also, belongs the physical stim- 

 ulus of the internal animal heat proper to the individual. The last- 

 named stimuli might be called vito-physical and vito-chemical. There 

 are other internal stimuli which may more properly be named vital, 

 for they are neither mechanical, physical, nor chemical. These are 

 the purely mental stimuli, which arise from consciousness and percep- 

 tion, and are either ideatibnal, emotional, or volitional. These, how- 

 ever, also originate, so far as the body is concerned, in peculiar states, 

 conditions or affections of the nervous centres, and in them only ; and, 

 as we shall hereafter see, their exercise is always associated with 

 chemical and electrical changes in the nervous molecules, and there- 

 fore they present likewise a vito-physical and vito-chemical aspect. 

 The stimuli which act on the body from without, have been character- 

 ized as objective, and those which act from within, as subjective stim- 

 uli ; but, as we shall explain in the chapter on Sensation, in speaking 

 of the so-called objective and subjective sensations, there is a certain 

 confusion in the use of these too much hackneyed and sciolistic 

 phrases. 



The external and internal stimuli, mechanical, physical, chemical, 

 and vito-physical, vito-chemical, or vital, also operate on, and excite, 

 the assimilative and plastic property of the tissues. Thus, without 

 external heat, no seed or germ is developed, heat being the so-called 

 " efficient cause" or " motive power," of all germination, development, 



