HIGH SCHOOL ZOOOLGY. 



23 



the anteiioi' boundary, wliile the posterior is occupied by another 

 unpaired bone, the basi-occipital. The latter articvdates with 

 the centrum of the first vertebra, and also affords support to a 

 bone of the shoulder-girdle (the supraclavicle or post-temporal) 

 which abuts against the strong transverse process of the fourth 

 vertebra and rests by two other prongs on the epi- and pterotic 

 projections. Between the mesethraoid and the basioccipital 

 the following bones are to be noticed ; the vomer, a T-shaped 

 bone, the transverse bar of wliich lies across the ventral surface 

 of the parethmoids, and the parasphenoid which continues back 

 the leg of the vomer in the middle line. All of the lateral projec- 

 tions seen from above are also to be seen from below. Certain 

 foramina furnish landmarks for the recognition of the other 

 l)ones to be seen on tliis surface : on either side of the basi- 

 occipital are the exoccipitals each with two foramina for tlio 

 escape of the 9th and 10th nerves. The sutures between the 

 basi- and exoccijutals, and the epi- and pterotics are occupied by 

 cartilage and are thus very distinct, and the pro-otics (whicli 

 are wedged in between the exoccii)itals and the pterotics) are 

 similarly marked out. In front of the pro-otics are the large 

 apertures by which the 5th and 7th cranial nerves escape, but 

 these apertures are also partly bounded by the sphenotic above, 

 the alisphenoid in front and the basisphenoid (an unpaired 

 bone partly concealed by the parasphenoid) towai'ds the middle 

 line. Between the alisphenoids and the orbitosphenoid, are 

 the optic foramina for the escape of the optic nerves ; the latter 

 bone is unpaired, and it forms a considerable part of the side 

 walls of the skull and also of its floor, where, however, it is 

 covered by the parasphenoid: it is also channelled by the 

 olfactory tracts on their way to the nasal sacs. Thus eleven 

 additional bones are to be seen from tliis aspect, of which five 

 are unpaired j viz., the basioccipital, tlie basi-, orbito-, and Dara- 

 sphenoids, and the vomer. 



