400 WORK OF THE PHYSIOLOGICAL STATION AT PARIS. 



If a series of successive photographs is taken upon a fixed jdate 

 during the act of opeiiiug or closing the mouth, the figure thus 

 obtained gives all the positions successively occupied by the bright 

 rod, and consequently all the displacements of the lower jaw itself. 

 Kow it will be seen tiiat, by reason of the sliding of the condyles of the 

 jaw in the glenoid cavities, the center of movement is found very low 

 upon the ascending ramus near the angle (fig. 3, PI. XLVIII). 



In movements of mastication this line takes different positions 

 according as we chew upon the incisor or upon the molar teeth. It has 

 still other movements during speech or in displacement of the chin 

 either forward or backward (tig. 4, PI. XLVIII). 



In these experiments chrono-photography gives us a true sketch of 

 the movements, and as the length of the bright rod is exactly equal to 

 that of the lower jaw its end traces out exactly the form of that i)art 

 of the glenoid surface upon which the condyle slides. This experiment 

 already shows us the necessary interdependence between the forms ot 

 organs and the forms of movements. It would be curious to follow out 

 in a series of animal species this anatomo-physiological parallel. 



It should be noted that these experiments were made under very 

 simi)le conditions and that it was not necessary to use the chrono- 

 photograi>hic apparatus. 



In this case it was of but little importance to find the velocity and 

 phases of movement of the lower jaw; it Avas only desired to determine 

 the successive positions of the jaw at the difterent degrees of opening 

 the mouth, a perfect equality in the intervals of time separating the 

 different images being unnecessary. Besides, these experiments were 

 made in winter, with a diffused and quite feeble light, a long exposure 

 (about one-fourth of a second) being necessary. The ])rocedure was 

 as follows: 



An ordinary photographic apparatus provided with a pneumatic shut- 

 ter Avas trained ui>on the subject of exi)eriment. Tliis subject having 

 his head tirndy supported from behind so as to fix it, opened his mouth 

 at several successive degrees of wideness, stopping an instant after 

 each, so that a photograph might be taken. The figure thus obtained 

 differs in no respect from that which thechrono-photographic apparatus 

 might have given, except that the intervals separating two successive 

 images are arbitrary. 



When it is wished to determine the trajectory of a point, an ordinary 

 photographic; ajiparatus is also sufficient. In this case the shutter is 

 held open during the entire movement, and if the i)oint is brilliant and 

 shown upon a dark background it traces its trajectory in the form of a 

 continuous line. It is in this manner that I was able to determine the 

 character of the movement of the atlas upon the axis, according to the 

 trajectory made by a brilliant point fixed upon the occiput. 



The movements of the ribs during respiration are determined in a 

 similar nnmuer. These movenu'uts are very complex because of the 



