2 1 "2 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



I. Occipital ring, of four bones, surrounding foramen magnum 

 at back of cranium la si-occipital below, an ex-occipital each side, 

 supra-occipital above. 



II. Parietal ring, of six bones basi-sphenoid below, a small 

 ali-sphenoid each side, two large parietals above. 



III. Frontal ring, of six bones pre-splienoid below, a small 

 orHto-sphenoid each side, two large frontah above. 



I. II. III. 



s. occ. pa. pa. fr. fr. 



ex. occ. ex. occ. al. s. al. s. or. s. or. s. 

 b. occ. b. s. pr. s. 



The cranium narrows anteriorly, and its front end is closed by 

 a median mesethmoid, which continues the direction of the basi- 

 cranial axis into the facial region. Cranial axis + facial axis 

 constitute the cranio-facial axis. The mesethmoid also assists in 

 the formation of the olfactory capsules. 



The auditory capsule is wedged in between the occipital and 

 parietal rings, the lateral gap between which is partly filled by a 

 squcwiotal bone. 



There are two smooth oval occipital condyles, one on each side of the 

 foramen magnum, mainly formed by the corresponding ex-occipitals, but 

 partly by the basi- occipital, which is a small, flat bone. Each ex-occipital 

 is produced downwards into a long par-occipital process, closely applied to 

 the hinder part of the tympanic bulla. The large, irregular supra- occipital 

 is raised into a prominent, shield-shaped elevation. The roof of the brain- 

 case is completed by inter-parietal, parietal, and frontal bones. The inter- 

 parietal is a small, unpaired bone, placed transversely, and partly shutting 

 out the parietals from union with the supra-occipital. These are large, 

 flattish bones, united together by a longitudinal sagittal suture, and with 

 the supra-occipital and inter- parietal by the transverse lamboidal suture. 

 A slender process runs downwards from the outer side of each. The 

 frontals, which cover the front of the rapidly-narrowing brain -case, are 

 united with each other by the median frontal suture, and with the parietals 

 by the transverse coronal suture. Each is very irregular in shape, and 

 possesses parts situated in very different planes. That part roofing the 

 brain-case is broadest behind, and tapers anteriorly, to form with its 

 fellow a sharp point. From the side of this part a plate-like orbital process 

 turns sharply downwards and inwards to form the upper part of the inner 

 orbital wall. Where it turns down, a large overhanging supra-orbital 

 process sticks out, which partly roofs the orbit. A process from the frontal 

 runs into the nasal region. The floor of the brain-case rapidly steepens in 

 front of the basi-occipital, and is formed in the middle line by the basi- 

 and pre-sphenoids, on each side by the ali- and orbito-sphenoids. The 

 basi-sphenoid is a small bone with a broad posterior end separated from the 



