222 HUMAN ANATOMY. 



translucency of the following parts will be very evident : the roofs of the orbits, one 

 or two uncertain points in the great wing of the sphenoid, one in the lower part of 

 the squamous portion just outside of the petro-squamous suture corresponding to 

 the glenoid fossa, the beginning of the basilar process, a varying portion of the 

 descending part of the groove for the lateral sinus, and nearly the whole of the floor 

 of the cerebellar fossa. A little rim of firm bone surrounds the foramen magnum 

 except in front. ^ 



THE FACE. 



This consists essentially of the framework of the jaws and of the orbital and 

 nasal cavities, as well as of certain accessory regions, the zygomatic and spheno- 

 maxillary fosses. Apart from features in the bones already described, the front 

 view shows the outline of the orbits, of the nasal opening, of the prominence of the 

 cheek, and of a vacant space left between the upper jaw and the ramus of the lower. 

 The foramina for the escape of the terminal branches of the three divisions of the 

 fifth nerve are very nearly in a vertical line, only the mental foramen is usually a 

 little lateral. The side view shows the zygomatic fossa below the arch and within 

 the ramus. 



The Orbit. — Although the base is quadrilateral, the orbital cavity is conical 

 rather than pyramidal, since its section a little behind the base is almost circular. 

 The upper margin of the entrance is formed by the frontal bone, which slants down- 

 ward to the very prominent external angular process, which affords great protection 

 to the eye. The suture with the malar can easily be felt in life, owing to the greater 

 projection of the upper bone. The outer border and the inner half of the lower are 

 made by the malar, which has a sharp orbital edge throughout. This is continued 

 by an ascending sharp edge of the superior maxillary into the front border of the 

 lachrymal canal, at the top of which it becomes indistinct. This is to be considered 

 the inner boundary ; but there is difficulty in accurately determining this border, for 

 if the upper border be followed down at the inner side, it will be seen to run to the 

 posterior edge of the lachrymal groove made by the ridge in the lachrymal bone. In 

 some skulls this is much the more evident border. The upper part of the inner border 

 is the only one that cannot easily be felt in life. The roof of the orbit is arched from 

 side to side and from before backward. It is overhung by the border, especially at 

 the outer angle, where it lodges the lachrymal gland. The injier wall, composed of 

 part of the ascending process of the maxilla, the lachrymal, the os planum of the eth- 

 moid, and part of the body of the sphenoid, is nearly vertical in front, but farther 

 back slants inward. The inner wall is frequently quite convex in the middle ; if this 

 condition is marked, it is probably pathological. There is an approach to an angle 

 between this surface and the upper. The two ethmoidal foramina are found above 

 the OS planum. The inner wall curves gradually into the inferior surface* formed 

 by the maxilla, and presenting the infra-orbital groove and canal. The outer wall 

 slants strongly inward, its lower border being internal to the upper. It is formed 

 by the malar bone in front and the great wing of the sphenoid behind. The back 

 part of the upper angle of the outer wall is occupied by the sphenoidal fissure, which 

 opens into the middle fossa of the skull, and the lower angle by the spheno-maxillary 

 fissure, separating the wing of the sphenoid from the maxilla ; the outer end of this 

 fissure, closed by the malar bone, opens into the zygomatic fossa. The optic fora- 

 me?i is at the posterior point of junction of the roof and the inner wall. The apex 

 of the orbit is at the inner end of the sphenoidal fissure. 



The axes of the orbits, if prolonged, cross each other -at the back of the sella 

 turcica at an angle of from 42° to 44°. The orbital axis is, therefore, very differ- 

 ent from the visual axis, which is antero-posterior. The former, moreover, runs 

 downward from the apex to the base, making an angle of from 15° to 20° with the 

 horizontal plane. 



The dimensions of the adult orbit vary with different observers and, no doubt, 

 in different localities. The depth is from forty to forty-five millimetres, the breadth 

 at the base is about forty millimetres, and the height about thirty-five millimetres 

 in males. In females the dimensions are rather less. 



The roof is thin and separates the orbit from the cranial cavity, except in 



