838 
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 entosternal element. This central piece is 
bounded anteriorly by the episternal bones, 
and posteriorly by another pair named the hyo- 
sternal, being as it were set in the centre of 
these lateral pieces as ina frame. The poste- 
rior half of the sternum consists of two pairs 
of elements, the larger and most anterior being 
designated as the hyposternal, whilst the pos- 
terior occupying the position of the xiphoid 
cartilage in the human skeleton are fitly deno- 
minated the «iphosternal 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 stili 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 (Clupea), and in these 
it is represented by a few azygos bones, on 
which the ribs abut inferiorly. 
In the Anourous Batrachia, 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- 
coidand clavicular bones that enter into the com- 
pen of the shoulder; the whole apparatus 
as been already figured in the article Ampur- 
Bia, (fig. 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 coracofd (d), the hypo- 
sternal bones (f'), and the xiphisternal elements 
OSSEOUS SYSTEM. (Comp. Awar.) 
oe 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 
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 : 
danger had the mesial sutures that exist in the 
sternum of the Tortoise been here ited. 
The pieces composing it are pointed out in fig. 
129, vol. i. p. 282, where the following elements: 
are delineated, viz. the entosternal (a), the hyo- — 
sternals (b), the hyposternals (c), and the xiphi= 
sternals (g). iy 
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, 
importance of this part of the osseous frame= 
work becomes considerably increased, and it is 
developed accordingly. 3 
Frequently connected with the sternum, t 
by no means to be ed as derivation 
from that bone, are the other importantelem 
of this part of the skeleton already noticed 
which, though entirely deficient in hum: 
subject and in the Mammalia generally, ar 
found in Birds and many Reptiles to be 
sential to the structure of the thorax. nes 
are the sternal or abdominal ribs (fig. 432, m 
a series of distinct bones in betwee 
the spinal ribs and the sides of the sternum, § 
as to form a complete osseous framework to th 
chest. In Fishes, as well as in the Ba chi 
and Ophidian Reptiles, these bones are abs 
lutely wanting, though in the Frogs and Toa 
the sternum is so large. But in the Sauria 
as, for instance, in the Crocodile, they | 
essential parts of the thoracic rece nd 
materially the movements requisite ) 
tion. In the Crocodile these ventral rit 
tend indeed much further backwards than 
dorsal ones that form the posterior bound 
of the thorax, being continued along the 
domen almost as far back as the pelvis 
bedded in the abdominal muscles, the at 
of which they doubtless materially strer 
In the higher Vertebrata, i.e. the Mama 
the sternal ribs are entirely represented b 
costal cartilages, and the abdominal w 
seem to be completely wanting; still 
seems but little doubt that, even in Ma 
lingering rudiments of ventral ribs are 
able in the tendinous intersections of the 
muscles of the abdomen. ~ 
In Birds the sternal ribs assume still 
importance as regards their effect in stre 
ening the thorax, and converting the ss 
fa 
i +f 
Ee - 
