836 



OSSEOUS SYSTEM. (Coup. AKAT.) 



the posterior margin of the os hyoides even in 

 the adult Frog, although they remain for a very 

 long while in a cartilaginous condition. The 

 two pieces a, a, then speedily become dimi- 

 nished in breadth, and the whole shape of the 

 os hyoides approximates that of the adult Frog, 

 the condition of the pieces marked a, a, forming 

 the chief difference between them. This at 

 length becomes gradually more slender and 

 elongated, assuming at last the shape delineated 

 in jig. 26, vol. i. (a), where the permanent and 

 complete condition of the os hyoides of this 

 Reptile is fully established. 



The different pieces composing the os hyoides 

 of a Bird having been already described and 

 figured (vide Article AVES. Jig. 151), it only 

 remains for us in this place to complete our 

 review of the hyoid apparatus by examining its 

 condition in the Mammiferous races, in which 

 it is found gradually to become stripped of 

 many parts that before entered so conspicuously 

 into the construction of this portion of the 

 osseous frame-work of the throat, and assume a 

 simplicity of structure that progressively assi- 

 milates to the shape it presents in the human 

 subject, in which alone indeed the name of 

 hyoides is at all applicable. In Man, observes 

 Geoffrey, the os hyoides is generally said to 

 consist of a body and four symmetric pairs of 

 branches or cornua. The anterior cornua (su- 

 perior when man stands in the erect posture) 

 are reduced to mere rudiments, but in those 

 Mammalia that have the head elongated these 

 anterior cornua are very largely developed, 

 appended from the sides of a special pair of 

 bones, the styloids, which, although in mankind 

 they are reduced to simple and almost useless 

 apophyses, consolidated with the temporal 

 bones in the generality of Mammifers, are very 

 large and important pieces, so connected with 

 the anterior cornua that they are frequently 

 regarded as being additional parts of the os 

 hyoides. But although in the human body 

 these apophyses are comparatively small, and 

 are respectively removed, as it were, to their 

 proper places, the styloids to the cranial bones, 

 and the anterior cornua to the os hyoides, they 

 perform the same office of connecting the liyoid 

 apparatus to the cranium in Man by the inter- 

 position of a cartilage, and in quadrupeds still 

 more effectually by an uninterrupted chain of 

 bones connected with each other. 



The posterior cornuu, each consisting of a 

 single piece, resemble each other in office at 

 least, in all the Mammalia, forming with the 

 body of the os hyoides a horse-shoe figure, to 

 which the larynx is appended. The body itself, 

 or central portion of the bone, although in the 

 human subject only represented by a slight 

 tuberosity, will be found in the Rodentia, 

 Ruminants, and more especially Solipeds, to 

 become very conspicuous, and in the last case 

 obviously distinct elements of the skeleton. In 

 these Mammalia indeed, the os hyoides is 

 found to consist of no fewer than nine pieces, 

 without enumerating the styloid bones ; a con- 

 dition of complexity almost approaching that 

 met with in the Birds and inferior Vertebrata. 



Leaving the consideration of the bones of the 



face, and those which enter into the compo- 

 sition of the hyo-branchial apparatus, which 

 may be all regarded as forming a succession of 

 arches depending from the sides of the cranial 

 vertebrae, of the transverse processes, of which 

 they are indeed regarded by some writers to be 

 real costal prolongations; the anatomist finds 

 a more or less extensive series of bones derived 

 from the sides of the spinal vertebrae, and fre- 

 quently arching downwards to enclose and pro- 

 tect the viscera either of the thorax or of the 

 abdomen, or of both. These lateral appen- 

 dages to the spinal column are invariably in 

 connection with the transverse processes, of 

 which in their simple forms they might seem to 

 be derivations, but when largely developed, as 

 for example in the thorax of Mammiferous 

 animals, they attain to a prodigious size, form- 

 ing, almost by themselves, the frame-work of 

 the thorax, and constituting the principal agents 

 employed in the performance of the mechanical 

 actions connected with the inspiration and ex- 

 piration of the air used for the purpose of 

 respiration. The position of the ribs thus 

 employed for the formation of a thorax is ex- 

 tremely variable in different races. In Man 

 and all other Mammalia, in obedience to a law 

 at present unexplained, they commence inva- 

 riably at the eighth spinal vertebra, counting 

 from the skull ; but in Birds the whole thorax 

 is removed much further backwards in order to 

 allow of the greater elongation of the neck. 



Besides the dorsal ribs thus derived from the 

 spine, and which exist alone in the human 

 subject and in Mammalia generally, another 

 series of additional elements is met with in 

 Reptiles and in Birds, which must be called 

 sternal ribs, and these, conjoined with the last, 

 enter into the composition of the thoracic cavity. 

 In Fishes only dorsal ribs are met with, and these 

 are connected by a simple articulating facet to the 

 sides of the bodies of the vertebrae placed imme- 

 diately above the abdominal cavity. Frequently, 

 however, the ribs of Fishes have supplementary 

 bones appended to them, which in the livino- 

 fish are embedded amongst the lateral muscles 

 of the body. Sometimes, indeed, these addi- 

 tional rib-like processes arise immediately from 

 the bodies of the vertebrae themselves, giving 

 an appearance of complexity to this portion of 

 the skeleton that is calculated to puzzle the 

 young osteologist. In the Cyprinidas and the 

 Herring tribe a small osseous piece is interposed 

 between the vertebra and the rib that is ap- 

 pended to it ; this is obviously a detached trans- 

 verse process. In the Batrachia dorsal ribs 

 only are found, and these, even when most 

 largely developed, are mere rudiments appended 

 to the ends of the transverse processes of the 

 vertebras. 



In Serpents likewise the enormously pro- 

 longed thorax is entirely made up of dorsal 

 elements, but these, existing as they do almost 

 along the whole length of the body, and being 

 moved by an elaborate muscular apparatus, 

 perform to a certain extent the office of loco- 

 motive organs. 



In the Chelonian and Saurian Reptiles the 

 construction of the thorax becomes much more 



