142 ANIMAL BIOLOGY. [Part I. 



The median segment is the parietal segment. Here the basal 

 bone answering to the basi-occipital is the basi-sphenoid (b. s.). 

 At its sides are two winglike bones, separable in the embryo, 

 but early ankylosed (united by bony union) the all-sphenoids 

 (a. s.). Besides its ascending process which enters into the 

 composition of the brain-case, each ali- sphenoid sends down- 

 wards two flattened laminae forming together a V with its point 

 directed forwards (see Fig. 46, pt. pa.). The median segment 

 is completed above by the parietals (pa.), two large roofing 

 bones which meet in the middle line. It is, however, incom- 

 plete at the sides, since the parietals do not reach down to the 

 ali-sphenoids. 



The anterior segment of the brain-case is the frontal segment. 

 The basal bone, answering to the basi-occipital and basi-sphenoid, 

 is the pre-sphenoid (p. s.). Separable from it in the embryo, 

 but early ankylosed with it, are two winglike bones, answering 

 to the ali-sphenoids. These orbito-sphenoids (44, o. s., and 45, or. s.) 

 form the posterior and superior boundaries of the large optic 

 foramen (44, 2, and 45, op. f.) which places the two orbits (con- 

 taining in the fresh state the eye-capsules) in communication, 

 and also opens into the brain-case. The pre-sphenoid forms its 

 anterior and inferior boundary, and then sends outwards ascend- 

 ing orbital processes (45, o. p. p.) which meet descending orbital 

 processes (o. p. /.), of thefrontals (fr.) large roofing bones which 

 complete this anterior segment of the brain-case. Each sends 

 out a supra-orbital process (s. p.f.) over the orbit. 



The anterior end of the brain-case is bounded by a bone 

 (cribriform plate, 44 A., c. r.) which is perforated like a sieve for 

 the olfactory nerve-threads (1). 



These three segments, occipital, parietal, and frontal, form 

 the greater part of the walls of the brain-case. Between the 

 occipital and parietal segments, however, there would be, were 

 no other bones developed, a great gap at the sides, and a 

 small gap in the roof. The posterior part of the gap on each 

 side is filled in with the penotic bone, which results from the 

 ossification of the auditory capsule. The gap in the roof is 

 filled in by a small oval interparietal (45, i.p.). Furthermore, at 



