838 



OSSEOUS SYSTEM. (Coup. ANAT.) 



ever, only in the Chelonian Reptiles that the 

 sternal bones present themselves in full complete- 

 ness, forming the broad shield or plastron that 

 defends the ventral aspect of the body in those 

 animals, which, being solidly connected on each 

 side with the dorsal plate or carapax, forms a 

 kind of box to shelter the whole body. 



The sternum, thus necessarily amplified to 

 the utmost, consists of no fewer than nine dis- 

 tinct elements, to all of which names have 

 been applied expressive of their position rela- 

 tive to each other. First, there is an azygos 

 element occupying the mesial or central 

 portion of the anterior part of the plastron, 

 which from its situation has been named 

 the ent asternal element. This central piece is 

 bounded anteriorly by the episternul bones, 

 and posteriorly by another pair named the hyo- 

 sternal, being as it were set in the centre of 

 these lateral pieces as in a frame. The poste- 

 rior half of the sternum consists of two pairs 

 of elements, the larger and most anterior being 

 designated as the hypostemal, whilst the pos- 

 terior occupying the position of the xiphoid 

 cartilage in the human skeleton are fitly deno- 

 minated the xiphosternal pieces, which are 

 united to each other and to the last-mentioned 

 pair by strong serrated sutures, as indeed are 

 all the elements above enumerated. 



In the aquatic Chelonians, the Turtles, the 

 same elements are met with entering into the 

 formation of the enormous sternnm, and their 

 positions with respect to each other are pre- 

 cisely similar ; but here, in order to lighten 

 the skeleton, large excavations are hollowed 

 out in the centre of the bone, so that the 

 three posterior pairs of sternal elements do 

 not meet in the mesian line; but in all other 

 respects their identity is at once evident. The 

 sternal apparatus, however, although most fre- 

 quently it enters largely into the formation of 

 the thorax, is still more nearly related to the 

 anterior extremity, with the movements of which 

 it is invariably intimately connected, and fre- 

 quently gives origin to the most important 

 muscles of locomotion. The sternum can 

 scarcely be said to exist in Fishes, with the ex- 

 ception of a very few (C/upeti), and in these 

 it is represented by a few azygos bones, on 

 which the ribs abut inferiorly. 



In the Anourous Eatrachia, as the Frog and 

 Toad, the sternum is remarkable as existing in 

 a very complete state of developement quite 

 independently of any other elements of the 

 thorax, seeing that in these animals there are 

 no ribs to be attached to it. It consists here 

 of a chain of bones, in which most of the 

 elements above enumerated are easily recog- 

 nizable, placed along the mesian line of the 

 breast, and supporting on either side the cora- 

 coid and clavicular bones that enter into the com- 

 position of the shoulder; the whole apparatus 

 has been already figured in the article AMPHI- 

 BIA, (Jig- 17,) where the episternal bones 

 forming the most anterior part of the series, 

 the hyosternal bones (e), the entosternal bone 

 (g), to the sides of which are attached both the 

 clavicle (c) and the coracoid (d), the ln/po- 

 sternal bones (f), and the xiphislernal elements 



completing the chain posteriorly, are all indi- 

 cated. 



The sternum of Birds is peculiar on account 

 of the prodigious developement of the azygos 

 or entosternal element of which it is principally 

 composed, a circumstance obviously intended 

 to strengthen this part of the skeleton, and 

 prevent the tearing asunder of the lateral por- 

 tions of the bone by the enormous strain of the 

 strong and massive muscles of flight, an acci- 

 dent of which there would have been great 

 danger had the mesial sutures that exist in the 

 sternum of the Tortoise been here permitted. 

 The pieces composing it are pointed out in fig. 

 129, vol. i. p. 232, where the following elements 

 are delineated, viz. the entosternal (), the lu/o- 

 sternals (b\ the hyposternals (c), and the jcip/ii- 

 sternals (g). 



The sternum of Mammalia becomes once 

 more reduced to its simplest form, consisting 

 of a chain of osseous pieces situated along the 

 mesian line on the anterior aspect of the thorax, 

 which they partially assist in forming. In some 

 races, however, as, for example, in the Mono- 

 tremata, so closely allied to Birds in all the 

 details of their economy, and also in Quadru- 

 peds possessing great power of using the an- 

 terior extremities either for flight or digging, 

 as, for example, the Bat and the Mole, the 

 importance of this part of the osseous frame- 

 work becomes considerably increased, and it is 

 developed accordingly. 



Frequently connected with the sternum, but 

 by no means to be regarded as derivations 

 from that bone, are the other important elements 

 of this part of the skeleton already noticed, 

 which, thoush entirely deficient in the human 

 subject and^ in the Mammalia generally, are 

 found in Birds and many Reptiles to be es- 

 sential to the structure of the thorax. These 

 are the sternal or abdominal ribs (Jig. 432, m) 

 a series of distinct bones interposed between 

 the spinal ribs and the sides of the sternum, so 

 as to form a complete osseous framework to the 

 chest. In Fishes, as well as in the Batrachian 

 and Ophidian Reptiles, these bones are abso- 

 lutely wanting, though in the Frogs and Tuud* 

 the sternum is so large. But in the Saurians, 

 as, for instance, in the Crocodile, they form 

 essential parts of the thoracic cavity, and aid 

 materially the movements requisite for respira- 

 tion. In the Crocodile these ventral ribs ex- 

 tend indeed much further backwards than the 

 dorsal ones that form the posterior boundaries 

 of the thorax, being continued along the ab- 

 domen almost as far back as the pelvis im- 

 bedded in the abdominal muscles, the action 

 of which they doubtless materially strengthen. 



In the higherVertebrata, i.e. the Mammalia, 

 the sternal ribs are entirely represented by the 

 costal cartilages, and the abdominal would 

 seem to be completely wanting; still there 

 seems but little doubt that, even in Man, the 

 lingering rudiments of ventral ribs are trace- 

 able in tHe tendinous intersections of the recti 

 muscles of the abdomen. 



In Birds the sternal ribs assume still more 

 importance as regards their effect in strength- 

 ening the thorax, and converting the thoracic 



