SKELETON. 57 



The formation of tlie posterior part of the side of the skull 

 is completed by the mastoid and parietal bones. The former 

 (12) projects outwards and backwards farther than the paroc- 

 cipital, forming the outer strong process of the side of the 

 cranium. This process lodges on its upper surface one of the 

 main ducts of the muciferous system, and affords the base 

 of articulation to a part of the hyomandibular. Its extre- 

 mity gives attachment to the strong tendon of the. dorso- 

 lateral muscles of the trunk. The parietals (7) are flat bones, 

 of comparatively much smaller extent than in higher Verte- 

 brates, and separated from each other by the anterior prolonga- 

 tion of the supra-occipital. 



The anterior wall of the brain-capsule (or the posterior of 

 the orbit) is formed by the orhitosphenoids (14), between which, 

 superiorly, the olfactory nerves, and inferiorly, the optic, pass 

 out of the cranium. In addition to this paired bone, the 

 Perch and many other fishes possess another single bone (15), — 

 the OS sphenoideum anterius of Cuvier, ethmoid of Owen, and 

 basispheTwid of Huxley ; it is Y-shaped, each lateral branch 

 being connected with an orbitosphenoid, whilst the lower 

 branch rests upon the long basal bone. 



A cartilage, the substance of which is thickest above the 

 vomer, and which extends as a narrow stripe along the inter- 

 orbital septum, represents the ethmoid of higher Vertebrata ; 

 the olfactory nerves run along, and finally perforate it. 



There remain, finally, the bones distinguishable on the 

 upper surface of the skull; the largest, extending from the nasal 

 cavities to the occipital, are the frontal bones (1), which also 

 form the upper margin of the orbit. The postfrontals (4) are 

 small bones placed on the supero-posterior angle of the orbit, 

 and serving as the point from which the infraorbital ring is 

 suspended. The prefrontals (2), also small, occupy the anterior 

 margin of the orbit. A pair of small tubiform bones (20), the 

 turhinals, occupy the foremost part of the snout, in front of 



