458 HENRY C. TRACY 



ventral plate {VPRO), which articulates posteriorly with the 

 ventral plate of the basisphenoid and with the posterior wings of 

 the parasphenoid below, to form the lateral wall of the eye- 

 muscle canal (fig. 8). This plate passes back from the medial 

 pillar on the orbital surface of the bone across the ventral 

 surface of the bony capsule in such a way that a small segment 

 of the capsule is exposed to the eye-muscle canal (fig. 8). The 

 foramen for the entrance of the carotid artery lies between the 

 ventral plate of the prootic and the parasphenoid wings. The 

 articulated medial plates of the prootics are joined with the 

 basisphenoid in front and the basioccipital behind, and thereby 

 form the roof of the eye-muscle canal. The ventral surface of 

 the remainder of the prootic bone forms a large part of the ven- 

 tral surface of the cranium. It is mainly taken up by a segment 

 of the osseous capsule which protrudes from the middle of its 

 surface. On each side of the front part of the capsule is a 

 foramen through which the branches of the facial nerve men- 

 tioned above make their exit. 



It is evident from the above description that a segment of the 

 prootic capsule projects into all the cavities of w^hich the prootic 

 bone forms a part. About a third of the surface of the capsule 

 contributes to the cerebral surface of the cranial floor; a segment 

 faces the orbit where it forms the floor of the ganglionic fossa; 

 about one-fifth of its surface protrudes from the ventral surface 

 of the cranial floor; a small segment is presented to the lateral 

 recess and another to the eye-muscle canal. A portion of the 

 cerebral segment limits the saccular cavity anteriorly. The 

 curvature of these segments is very little altered by its attach- 

 ment to the parts of the bone in which it lies. 



There are two openings in this capsule; the posterior (fig. 5) 

 is the mouth of the canal through which the swimbladder diver- 

 ticulum enters the interior of the capsule. The prootic portion 

 of this canal begins at the posterior lateral angle of the prootic 

 and proceeds obliquely through the thick part of the bone 

 lateral to the saccular portion of the auditory recess (fig. 10). 



The other opening or fenestra is in the cerebral segment of the 

 capsule in that part of its curvature which looks posteriorly 



