378 SENSITIVE SYSTEM. 



We have formerly examined this, and found it to possess consi- 

 derable density, firmness, and resistance, and to be thickest at 

 the external and superior parts; and we now perceive that it was 

 made so because there it has to supply the place of bone. The 

 socket formed by the sheath is described by some as conical, by 

 others as pyramidal ; perhaps it bears the nearest approach to 

 the latter form, though, like other anatomical comparisons to 

 geometrical figures, it will be found not to possess mathematical 

 precision : for the basis of the pyramid, instead of being equi- 

 distant at every point from the apex, is obliquely truncated in a 

 direction outward and downward ; to which maybe added, there 

 is a prominence of the boundary edge next the nose that contri- 

 butes with this truncation to give the eye a sunken appearance 

 on that side, while the outer partof the globe projects beyond the 

 socket. The orbital ridge has a quadrilateral figure : the exter- 

 nal side is the longest, the inferior one the shortest, the vertical 

 to the transverse diameter being in the ratio of seven to six : the 

 line of the internal side is interrupted by a notch. The axis of 

 the orbit directs a horizontal line more outward than forward, 

 more forward than downward : this line intersects another hori- 

 zontal line drawn directly forward at an angle of about 70°, and 

 one drawn directly outward at about 20° : the inclination down- 

 ward, however, will of course vary with the position of the head. 

 The axis of the eyeball is not exactly that of the orbit, though it 

 likewise takes a direction more outward than forward ; for in the 

 ordinary position of the head, it is perfectly horizontal, and, in 

 consequence of the prominence of the eye next the temple, in- 

 clines more forward than the orbital axis, probably by 10° : the 

 motions of the eye will, in course, vary its axis considerably, and 

 especially in the lateral direction. In fact, the eyes, placed as 

 they are in the head, command nearly a whole sphere of vision : 

 the horse consequently is amply provided with visual means for 

 seeking food and avoiding danger. 



Situation and Connexion. — The eyeball is placed within the 

 anterior or more capacious part of the orbit, nearer to the frontal 

 than the temporal side, with a degree of prominence peculiar to 

 the individual, and within certain limits variable at his will. In 

 front, the ball is suspended by the eyelids ; laterally and pos- 

 teriorly, it is slung by its seven muscles, and posteriorly it is also 

 retained by the optic nerve : which muscles and nerve, together 

 with its bloodvessels, are, as we have seen, enveloped in fat ; and 

 this fat forms a bolster for the globe (as has also been pointed 

 out), maintaining it in a due state of advancement, and materially 

 assisting in preserving its proper position, and giving steadiness 

 to its movements. 



