June 1, 1900.] 



SCIENCE. 



855 



so far from biological naivete as these, 

 there exists a psychophysical principle 

 which can be concisely stated as follows : 

 In general, states of pleasant co7isciousness are 

 correlated in the body by contraction of mus- 

 cles classed as extensor, and, conversely, states 

 of unpleasantness with contraction of flexor 

 muscles. This is an ancient and more or 

 less popular hypothesis, but usually in a 

 form where bodily expansion and contrac- 

 tion are suggested rather than muscular 

 extension and flexion respectively, a ten- 

 dency indeed which is to be observed in 

 very many animals from the amoeba, the 

 type, upward in morphologic complexity. 

 This expansion-flexion tendency, so to call 

 it for convenience, being then an inherent 

 opposition in the neuro-muscular structure 

 of animal organism in general, its relations 

 to the hitherto mysterious phenomena of 

 the smile are of a certain general interest. 

 Now it appears, in short, that the smile and 

 laugh are in fact fully explained by this general 

 biologic principle, as will become clear from 

 the following considerations : 



Even as the afiective principles are mani- 

 fested as early as animal life itself, so must 

 one search very early in the morphologic 

 changes of the individual, very likely indeed 

 in the early stages of the embryo, for the 

 forms which normally determine adult affec- 

 tive structure and function. Function is 

 properly more primary than form, and one 

 generation determines by its habits to some 

 extent invariably the structure of the next ; 

 we should look, then, to the early embryo 

 for the direct correlation of the functions 

 which are to obtain in the adult life. At 

 any rate, in the fcetus we find conditions, 

 more plainly marked than in later states, 

 which make adult functional relations clear. 

 Although the ' primitive streak ' is at first 

 necessarily straight, folding begins very 

 early, and at the end of the first month, in 

 the human fcetus, the embryo is bent upon 

 itself to a very marked degree. The curve 



which forms in the region afterwards the 

 lower cervical is for our purpose the most 

 significant, although the growth forward of 

 the anterior lobes helps in the case of man 

 to make this antero-posterior bending more 

 marked and physiologically more signifi- 

 cant. This curvature remains quite patent 

 in the adult in all the canals which extend 

 from before backwards in the head : First, 

 in the ventricles of the brain from the third 

 to the ' sixth ' down the spinal cord. Second, 

 from the nostrils upward and backward and 

 then downward into the trachea. Third, 

 from the mouth similarly upward and back- 

 ward and downward into the oesophagus. 

 All these in a proper philosophic sense out- 

 line even in the adult forms the curve 

 which, at the period of individual develop- 

 ment when the functions of the muscular 

 system were being firmly established in the 

 formative plan of the organism, is even more 

 plainly apparent. In the strongly progna- 

 thous type of structure these conditions are 

 all emphasized, as they are in animals with 

 long probosces which these muscles in part 

 serve to extend. It is at this very early 

 formative period that the neuro-muscular 

 system is beginning to take on its basal 

 opposition of flexion and extension, the 

 large muscle of the scalp, the Occipito 

 frontalis, being the strikingly complete type 

 of this far-reaching functional division, for 

 the frontal half draws directly backward in 

 a way to tend to extend the face and the 

 head. 



On this same dual principle do all the 

 facial muscles act, as may be seen from a 

 brief technical rehearsal of the most im- 

 portant of them and their functions. The 

 Risorius, the ' laughing muscle,' draws 

 upward and backward the corners of the 

 mouth ; the Zygomatici have a similar action 

 on the lower jaw ; the Levator labii supe- 

 rioris, as its name perhaps sufiiciently im- 

 plies, lifts the upper lip, as does the Levator 

 labii superioris et alseque nasi, and in addi- 



