OSSEOUS SYSTEM. (COMP. ANAT.) 



833 



The styloid &0wes(29) 3 mere rudiments 

 in the human skeleton, anchylosed with 

 the rest of the temporal bone, of which 

 they are called the " styloid process," 

 in the water-breathing Vertebrata are 

 distinct pieces interposed between the os 

 hyoides and the base of the skull, serving 

 to unite the former to the latter. 



The symplectic bones (31) seem to be 

 peculiar to Fishes ; they accompany the 

 transverse, and assist in connecting the 

 articulation of the lower jaw with the 

 pterygo-palatine apparatus. 



The lower jaw, although in the adult 

 human subject formed of a single piece, 

 in the fatus consists of two lateral halves 

 united by a symphysis, as it is perma- 

 nently in many of the lower Quadru- 

 peds. In Reptiles and Fishes, how- 

 ever, each half consists of numerous 

 pieces, to which distinct names have 

 been given by the comparative anato- 

 mist. In the Crocodile and most reptiles 

 there are six in number, viz. the dental 

 portion (34), in which are situated all 

 the alveoli of the teeth, uniting with its 

 fellow to form the symphysis of the jaw- 



The opercular, covering almost all the inner 

 aspect of the jaw except in front. 



The angular (36) and the supra-angular (35), 

 placed one above the other, reaching quite to 

 the posterior extremity of the jaw. In the 

 Crocodile they leave between them a conside- 

 rable space occupied anteriorly by the end of 

 the dental portion, and then by a large oval 

 aperture. 



The articular (e), bearing the articular pro- 

 cess, whereby the jaw is connected with the 

 skull. Likewise another small and unimpor- 

 tant plate of bone sometimes seen on the inner 

 aspect of the inferior maxilla. 



In the Chondropterygious Fishes the lower 

 jaw is made up of only one bone, the arti- 

 cular, upon which the teeth are affixed : rudi- 



Fig. 446. 



Fig. 445. 



Hitman Skull* 



VOL. in. 



Human Skull. 



ments of the others are, however, met with 

 imbedded in the flesh beneath the skin. 



The hyo-branchial apparatus. The osseous 

 framework to which in the human subject the 

 name of os hyoides has been appropriated, from 

 the trivial circumstance that in the simple con- 

 dition under which it presents itself in man 

 it resembles the Greek letter v, is found in the 

 lower Vertebrata to be permanently composed 

 of very numerous pieces, which are made sub- 

 servient to respiration, and from their size and 

 number render the whole apparatus, which 

 they assist in forming, really worthy of the 

 name of an anterior thorax. The hyoid svstern 

 of bones may indeed be regarded as being in 

 some respects vicarious in function with the 

 true thorax, the former belonging especially to 

 the aquatic, the other to the aerial mode of 

 respiration ; whilst, therefore, as 

 in Fishes, the gills form the only 

 means of breathing, and the branchial 

 arches exist in their full state of 

 developement, the hyo-branchial ap- 

 paratus is complete and preponde- 

 rates in importance over the thorax ; 

 but, in proportion as pulmonary 

 respiration is established, as we 

 ascend the scale of animal existence, 

 the thoracic system of bones as- 

 sumes the principal duties con- 

 nected with the inspiration and 

 expiration of air, and the os hyoides 

 dwindles into a very rudimentary 

 condition. The above circum- 

 stances however, interesting as they 

 are when a mere comparison is 

 instituted between the hyoid bones 

 of various animals as to their com- 

 position when in the adult state, 

 assume additional importance when 

 we reflect that all the higher Ver- 

 tebrata possess in the earlier stages 

 3 it 



