80 



ZOOLOGY 



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 (TO) 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) 



FIG. 765. A, diagram of bony skull in sagittal section ; B, transverse section of occipital region ; 

 C, of parietal region ; D, of frontal region ; E, of ethmoidal region. Cartilaginous parts are 

 dotted ; replacing bones are marked in thick type, investing bones in italics, mck. c. Meckel's 

 cartilage ; Nc. 1 10, foramina for cerebral nerves ; r. rostrum ; s. t. sella turcica or pituitary 

 fossa. Replacing bones AL. SPH. alisphenoid ; ART. articular ; B. BR. basi-branchial ; 

 B. HY. basi-hyal ; B. OC. basi-occipital ; B. SPH. basi-sphenoid ; C. BR. cerato-bran- 

 chial ; C. HY. cerato-hyal ; EC. ETH. ecto-ethmoid ; EP. BR. epi-branchial ; EP. HY. 



epi-hyal ; EX. OC. ex-occipital ; H. BR. hypo-branchial ; H. HY. hypo-hyal ; HY.M. 

 hyornandibular ; M. ETH. mesethmoid ; OP.OT. opisthotic ; OR. SPH. orbito-sphe- 

 nuid ; PAL. palatine ; PH. BR. pharyugo-branchial ; PR.OT. pro-otic ; PR. SPI 



pre-sphenoid ; PTG. pterygoid ; QXT. quadrate ; S. OC. supra-occipital. Investing bones 

 J)NT. dentary ; FR. frontal; MX. maxilla; NA. nasal; PA. parietal; PA.SPH. parasphe- 

 noid ; PMX. premaxilla ; SQ. squamosal ; VO. vomer. 



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 alisphenoids 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 



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

 of the most anterior vertebrse. 



