ON THE VERTEBRATE SKELETON. 299 



from each other by the extension and mutual junction at the median line 

 of the occipital and frontal spines. A specimen of this, in a species of 

 Cebus, which repeats the common modification of the parts in fishes, is pre- 

 served in the museum of the Royal College of Surgeons. The par -apophysis 

 (s) always commences as an autogenous element by a distinct centre of ossi- 

 fication, as shown in the human foetus, fig. 11,8; it speedily coalesces with 

 the petrosal, but otherwise retains its individuality in some of the lower mam- 

 mals, as e.g. in the echidna (fig. 12, s) : or it coalesces with the curtailed 

 frontal pleurapophysis 28, or with the maxillary appendage 27, or with both 

 these and the pleurapophysis of its own vertebra (3s), when the complex 

 'temporal bone' of anthropotomy is the result. In most mammals the pleur- 

 apophysis (38) retains its primitive independency and rib-like form, with 

 usually the 'head' and 'tubercle'; but by reason of its arrested growth it 

 has been called ' styloid' bone or process. Sometimes it is separated from 

 the short hccmapophysis, 40, by a long ligamentous tract, sometimes it is imme- 

 diately articulated with it, or by an intervening piece. The hccmal spine, 41,* 

 is usually small, but thick and always single. The rudiments of hypobranchial 

 elements (40) are retained as diverging appendages of the parieto-haemal arch 

 in all mammals, and have received the special names of ' posterior cornua,' 

 or ' thyrohyals,' from their subservient relationship to the larynx. 



In the frontal segment the centrum, 9, and neurapophyses, 10, very early 

 coalesce. Two separate osseous centres mark out the body (fig. 26, C, 9), 

 and each neurapophysis has two distinct centres (ib. 10, 10), the optic foramina 

 (op) being first surrounded by the course of the ossification from these 

 points. The superior development of the neurapophysial plates (10), as com- 

 pared with those of the parietal vertebra (g), in most mammals, harmonizes 

 with the greater development of the prosencephalon ; but the chief bulk of 

 this segment of the brain is protected by the expanded spines of the frontal (11) 

 and parietal(7) vertebrae, and by the intercalated squamosals (27). And the ap- 

 pendicular piece (27) not onlyusurpssome of the functions of the proper cranial 

 neurapophyses, but, likewise, the normal office of the frontal pleurapophysis 

 (23), in the support, viz. of the distal elements of the haemal arch (29, 32), 

 which now articulate directly with 27, in place of 28 as in all oviparous verte- 

 brates. The true pleurapophysis of the frontal vertebra (2s) is almost re- 

 stricted in the mammalian class to functions in subserviency to the organ 

 of hearing, is sometimes swollen into a large bulla ossea, like the parapophyses 

 and pleurapophyses of the cervical vertebrae of Cobitis ; it is sometimes pro- 

 duced into along auditory tube, and sometimes reduced to the ring supporting 

 the tympanic membrane. Yet, under all these changes, since its special ho- 

 mology is demonstrable with 28 in the bird (fig. 23) and crocodile (tig. 22) as 

 well as with the ideologically compound bone, 23 a, b, c, d, in the fish (fig. £.), 

 so likewise must its general homology, which is so plainly manifested in 

 the fish, be equally recognised. The frontal Jtcemapophysis (fig. 24, 29, 30), 

 and the corresponding half of the haemal spine (ib. 32) are connate on each 

 side in all mammals, and become confluent at H in, in most. The haemal 

 arch of the frontal segment of the skull, as in other air-breathing vertebrates, 

 has no diverging appendage, unless the tympanic otosteals be so regarded, 

 an idea which is not borne out by their development. 



The nasal segment (N iv, H iv) is chiefly complicated by the confluence of 

 parts of the enormously developed olfactory capsules (is) in the mammalian 

 class, and its typical character is masked by the compression and mutual coa- 

 lescence of the neurapophyses, 14. The centrum is usually much elongated, 

 as at 13, and soon coalesces with both neurapophyses (14) and nasal capsules 

 in the hog. The neural spine (15) is usually divided, but is sometimes single, 



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