Chap. VIII.] 



THE SKELETON. 



147 



to the membrane of the fenestra ovalis is a little stirrup-shaped 

 bone, the stapes (Fig. 47, sta.), bearing a little knob at the 

 summit of its arch, to which there articulates, by means of a 

 minute intermediate orbicular bone, a little bone, the incus (in.), 

 shaped somewhat like a two-fanged tooth. The fangs are not 

 of the same length; that articulated with the stapes is the 

 longer (long crus), while the shorter (short crus) articulates with 

 the overhanging wall of the periotic. The crown of the tooth- 

 like incus is fashioned into the shape 

 of a saddle, with which articulates 

 the head of another small bone, the 

 malleus (mal.), the shaft of which is 

 affixed to the tympanic membrane. 

 Thus the auditory ossicles form a 

 chain of bones between the tympanic 

 membrane on the one hand, and the 

 fenestra ovalis on the other. 



(4.) The Optic Chamber. The capsule 

 of the eye is not ossified, nor is it fused 

 with the bones of the skull by cartilaginous union. It is 

 enclosed in the orbit. 



In the rabbit's skull there will be seen, on each side, a single 

 large cavity, the two communicating by means of the optic 

 foramen. [But in the cat's skull, or the dog's, this large cavity 

 on each side will be seen to be imperfectly divided into two, of 

 which the anterior is for the lodgment of the eye-capsule, and 

 is the true orbit, while through the posterior, the temporal fossa, 

 pass the great muscles for the lower jaw. In the human skull 

 these are quite distinct.] The orbital recess of the rabbit is 

 thus a common orbit and temporal fossa. 



Its inner boundary is the wall of the brain-case, the frontal 

 sending out a supra-orbital process (see Fig. 45, s.p.f., p. 141). 

 Posteriorly the squamosal (sq.) sends out a curved and distally 

 expanded zygomatic process (z. p. sq.), running at first outwards 

 and then downwards, to be applied to and form part of a lateral 

 bar or zygoma (zy.). This is composed of a single bone forming 

 a process of the large maxilla. In the new-born rabbit, however, 



FlG ' 47 - AuDITORY 0ss ' CLES - 



