248 THE INTEGUMENT AND ORGANS OF 



head. It is inclined to that axis in the animal. The value 

 of the angle formed gives the angular value of inclination 

 of the head. This suture indicates in animals the proper 

 position of the orbit and head ; when it is horizontal, the head 

 is in the position of normal obliquity. 



c. The human orbit presents three walls, and in like 

 manner, when perfect in the mammalian series, it consists 

 of three walls, the inner, outer, and superior wall. The only 

 orbit in which the inner walls of opposite orbits converge 

 forwards is the human, so that in it only, when the orbits 

 are looked into directly from the front, can the two optic 

 foramina be seen at one time. 



d. The planes of the apertures of the opposite orbits in 

 the mammalian series are inclined forwards towards one 

 another, at an angle which increases as the series descends. 

 These planes of the orbital apertures are also in a descending 

 order inclined upwards and inwards. In man the planes 

 of the orbital apertures coalesce. They are transverse and 

 vertical. The margin of the human orbit presents at the 

 same time an oblique double curvature, not observable in the 

 ape or in any lower form. The margin is co-ordinated with 

 the great amount of movement of the human eyeball, especially 

 through the oblique muscles. An inclination of the orbital 

 plane, upwards and inwards, takes place in the mammalian 

 series, but not in man. 



c. The human orbit presents the most elongated form, and 

 most extended outer wall. 



/. These orbital peculiarities of the human skull are spe- 

 cial provisions for a greater freedom and extent of visual direc- 

 tions, but more especially to provide for a more perfect binocu- 

 lar vision. 



g. The semicircular canals of the organ of hearing are 

 connected with the sense of direction of sound. They are 

 situated at right angles to each other. No one is situated 



