OSSEOUS SYSTEM. (CoMP. ANAT.) 



(fig. 438, 1) united together. In die Siren 

 and Proteus, however, the principal f'rontals are 

 divided as in other Reptiles. In all Birds and 

 Mammalia these bones become at an early 

 period confused with the anterior and posterior 

 frontals and ultimately with each other, so as 

 to form but one piece, the os J'rmitis of man; 

 nevertheless, even in the human foetus, they are 

 separated by a suture which, in the lower 

 Mammalia and also in the human subject, is 

 not unfrequently persLstentto alate period of life. 



The anterior frontals (2) in the osseous 

 Fishes bound the orbit anteriorly. Between 

 these bones pass out the olfactory nerves, but 

 they are not always distinctly recognisable, 

 being occasionally permanently cariilaginous. 

 In Reptiles these bones are generally distinct, 

 but in Birds and Mammalia they coalesce with 

 the preceding. 



The poster ior frontals (4) form the posterior 

 margin of the orbit, and are present in Fishes 

 and the Reptilia, but in Birds and Mammifers 

 they are no longer recognisable as distinct 

 bones. 



The parietal bones (7) are placed behind the 

 frontals; but these bones do not always touch 

 each other, being separated, especially in Fishes, 

 by the interposition of an azygos bone from 

 w'hich projects the occipital spine, which is 

 frequently, more especially in Fishes, of very 

 good size : this impair bone, the interpurietul 

 of some authors, is in reality the representative 

 of the superior occipitals of Cuvier (supra- 

 occipitals, Owen ;) (8) and in some Fishes, es- 

 pecially in the Si/uri, where the parietals are 

 totally wanting, their place is supplied by the 

 enormous developement of this element of the 

 skeleton. 



The external occipitals (9) contribute to form 



Fig 440. 



Section of the skull of Turtle ( Testttdo Mi/as). 



829 



the lateral portions of the occipital region of 

 the skull, in conjunction with two other pieces 

 called 



The lateral occipitals (10), which partially 

 bound the foramen magnum. 



The inferior occipital or basi/ar bone (5) is 

 that which invariably is articulated to the body 

 of the first cervical vertebra, but occasionally 

 in Fishes there are two additional articulations 

 connecting the cranium to the spinal column 

 formed by the lower portions of the lateral 

 occipitals. All these elements of the so-called 

 occipital bone of the human cranium remain 

 permanently distinct in Fishes and Reptiles, and 

 even in the foetal condition of Birds and Mam- 

 mals are more or less recognisable ; but they 

 soon coalesce into one large piece that enters 

 largely into the formation of the cranial box, 

 and constitute the first or occipital cranial 

 vertebra, as has been already seen. 



The sphenoid (6) invariably occupies the cen- 

 tral portion of the base of the cranium, and in 

 Fishes and Birds is prolonged anteriorly into a 

 lengthy process which passes beneath the inter- 

 orbital septum, which, in these classes of Verte- 

 brata, remains most frequently membranous. 



The alar bones (11; alt-sp/ienoid, Owen,) 

 represented in the human subject by the greater 

 alae of the sphenoid, are in reality distinct 

 elements of the cranium, and are recognizable 

 by several important characters, especially by 

 their position being joined by suture to the 

 posterior frontral, and, conjointly with the latter 

 bone, to the temporal. Moreover, through 

 these bones the two posterior divisions of the 

 fifth pair of nerves always pass out from the 

 skull. In Fishes and Reptiles they are im- 

 portant pieces and quite detached from the 

 sphenoid. 



The squamo-temporal bones, Owen (mastoid 

 bones, Cuv. : 12) in Fishes are manifestly the 

 representatives of the bones so named in Rep- 

 tiles. They contribute in conjunction with the 

 posterior frontal, and occasionally with the alar, 

 to furnish the articular surface that sustains the 

 first bone of the palatine and tympanic appa- 

 ratus, or, in other words, of the masto-tem- 

 poral (23). 



The petro-teniporal bones, Owen, (13) are in 

 Fishes placed between the mastoid, the lateral, 

 occipital, and the alar bones. They are gene- 

 rally of small size, but occasionally, as in the 

 Gadid(E, very largely developed, descending to 

 reach the inferior occipital and the sphenoid. 

 On the other hand, they are frequently entirely 

 wanting, as, for instance, in the Pike, the Carp, 

 and the Eel. 



In all the Reptilia the petro-temporal bones 

 are recognizable as distinct pieces forming part 

 of the cranial box, and become interesting, 

 inasmuch as it is in them that the auditory 

 apparatus is lodged. 



In Birds and Mammalia, however, the 

 petrous bones become at an early period 

 inseparably soldered to the other pieces, form- 

 ing the so-called " temporal bone." 



The ingrassial bones (14), as they have been 

 named by Geoffroy, are, in the human subject, 

 regarded as portions of the sphenoid, although 



