78 ZOOLOGY SECT 



are connected and complete the side-walls of the interorbital region. 

 The basi-occipital, basi-sphenoid, and pre-sphenoid together form 

 the basis cranii of the bony skull. A vertical plate of bone, the 

 mesethmoid (M. ETH.), appears in the posterior portion of the 

 cartilage of the same name, and the outer walls of the olfactory 

 capsules may be ossified by paired ecto-ethmoids (E, EC. ETH.). 



So far, it will be seen, the cranial cavity has its hinder region 

 alone roofed over by bone, viz. by the supra-occipital : for the rest 

 of it the replacing bones furnish floor and side-walls only. This 

 deficiency is made good by two pairs of investing bones, the 

 parietals (PA), formed immediately in front of the supra-occipital 

 and usually articulating below with the ali-sphenoids, and the 

 frontals (FR), placed in front of the parietals, and often connected 

 below with the orbito-sphenoids. A pair of nasals (NA) are 

 developed above the olfactory capsules and immediately in advance 

 of the frontals ; and below the base of the skull two important 

 investing bones make their appearance, the vomer (vo) which 

 may be double in front, and the para-sphenoid (PA. SPH) 

 behind. 



The result of the peculiar arrangement of replacing and invest- 

 ing bones just described is that the brain-case, in becoming 

 ossified, acquires a kind of secondary segmentation, being clearly 

 divisible in the higher groups, and especially in the Mammalia, 

 into three quasi-segments. These are the occipital segment (B) 

 formed by the basi-occipital below, the ex-occipitals at the sides, 

 and the supra-occipital above 1 ; the parietal segment (C), formed by 

 the basi-sphenoid below, the ali-sphenoids laterally, and the parietals 

 above ; and the frontal segment (D) constituted by the pre-sphenoid 

 below, the orbito-sphenoids on either side, and the frontals above. 

 It must be observed that this segmentation of the cranium is quite 

 independent of the primary segmentation of the head, which is 

 determined by the presence of myomeres and by the relations of 

 the cerebral nerves. 



The cranial bones have constant relations to the cerebral nerves. 

 The olfactory nerves (A, Nv. 1) pass out one on either side of the 

 mesethmoid, the optic nerves (Nv. 2) through or immediately 

 behind the orbito-sphenoids, the fifth nerves (Nv. <5) through or 

 immediately behind the ali-sphenoids, and the tenth nerves (Nv. 10) 

 through or immediately in front of the ex-occipitals. 



It will be seen that a clear distinction can be drawn between 

 the primary cranium or chondrocranium, formed by the fusion of the 

 parachordals, auditory capsules, and trabeculae, and consisting of an 

 undivided mass of cartilage more or less replaced by bones, and 

 the secondary cranium or osteocranium, modified by the super- 

 addition of investing bones. 



1 With the occipital segment in many Fishes are amalgamated one or 

 several of the most anterior vertebra?. 



