CH. III.] SELF-DEFENCE. 155 



self, reminds us of this condition antagonistic to disagreeable 

 weariness, it leads us to the instinct for exhilaration, and we 

 stretch and yawn with the person. 



iii. It still remains to state specially, with reference to the 

 instinct for repose, that the physical and mechanical forces of 

 the machines of animal bodies, as also the vis nervosa on which 

 nerve- actions are dependent, in so far as they are not also 

 at the same time sentient actions, are not subject to this law 

 of nature, namely, that their uninterrupted activity shall cause 

 unpleasant external sensations, and, consequently, induce the 

 stimuli of the instinct for repose and sleep. The formation of 

 the blood, and its continuous internal movement, together with 

 its circulation ; the working of the elasticity and other purely 

 physical and mechanical forces of the machines ; nay, all those 

 processes of the mechanical machines which during the waking 

 state are sentient actions, but at the same time may be and 

 commonly are, even during the waking state, purely nerve- 

 actions (183), as, for example, the movements of the heart, sto- 

 mach, intestines, and various muscles, particularly the muscles 

 of respiration (285), all these, as such, are never accompanied 

 by a sensation of fatigue, never excite the instinct for repose, 

 never stand in need of repose, are never changed by this 

 instinct, nor directly by its satisfaction during the deepest 

 sleep, but go on continuously, and take no further part in it 

 unless they are at the same time sentient actions, or indirectly 

 influenced through the general connection of all the forces of 

 the animal (Haller's '^ Physiology,' § 579). On these principles, 

 all the phenomena of the animal economy, which depend upon 

 the sensational stimulus to sleep, on the instinct itself, and on 

 the satisfaction of the instinct, or the act of sleeping, may be 

 very readily explained. 



The Instinct of Self-Defence. 



288. Just as nature has supplied every animal with me- 

 chanical machines (organs), which serve as the instruments of 

 the instincts for self-preservation, for they receive both the 

 external sensational stimuli that excite these instincts and the 

 external sensations that satisfy them (281 — 285) ; nay, just as 

 every creature is taught and enjoined by other instincts to 



