OSSEOUS SYSTEM. (Comp. Anat.) 
complicated hy the developement of additional 
elements hereafter to be described, and in these 
tribes the examination of the dorsal ribs ex- 
hibits to the osteologist several points of very 
great interest relative to this portion of the 
skeleton. In the Crocodile, for example, the 
elements derived from the vertebre present 
every gradation of form between the simplest 
and most complex condition of the costal 
pieces. The hinder ones are loose and floating, 
being mere appendages to the transverse pro- 
cesses, to the ends of which they are fixed as in 
the Batrachia, by a simple undivided articu- 
lation; but as we advance forwards from this 
point along the true thorax, their connection 
with the vertebrae becomes progressively changed 
through a series of most beautiful gradations 
of form; the head of the rib becomes slowly 
_ divided into two distinct articulating surfaces, 
both of which are at first attached to ‘the trans- 
verse process of the corresponding vertebra, 
but more anteriorly the bifurcation of the head 
of the rib being completed, one division be- 
comes attached to the body of the vertebra, 
while the other, the tubercle, is fixed to the 
transverse process, and every gradation inter- 
mediate between the two extremes of structure 
presented in this portion of the skeleton is 
thus exhibited in the same animal. But dorsal 
ribs are developed in the Crocodile anterior to 
the thorax and with a very different office. costal 
"appendages (fig. 432, 0.) being attached to all 
the transverse processes of the cervical vertebra. 
These, instead of being prolonged downwards, 
spread out anteriorly and posteriorly, assuming 
the shape of the letter T, and forming a con- 
tinuous chain of bones, that trammels the lateral 
movements of the neck, but at the same time 
affords ample surface for the attachment of the 
unusually strong muscles of this Reptile’s neck. 
The dorsal ribs of the Chelonian Reptiles are 
equally interesting on account of the strange 
modification in the manner of their connection 
with the spine, whereby they are absolutely 
brought quite to the exterior of the body, and 
in the Tortoises so completely united by suture 
to the spinous processes of the vertebrae, and 
likewise to each other, as to form the greater 
portion of the dorsal shield or carapax peculiar 
to these races. In order to effect this total 
change in the position of the costal elements of 
_ the skeleton, the anatomist finds to his asto- 
nishment that very simple arrangements are 
necessary. The neuro-spinal apophyses of the 
vertebre are prodigiously developed and spread 
out into broad flat osseous plates firmly con- 
_ nected with each other and with the tubercles 
of the ribs by means of their broad serrated 
_ tiargins; this being accomplished, the usual 
_ attachments between the head of the rib and 
oth spine become unnecessary ; the bodies of 
= 
a 
Ne a ee 
_ the vertebrae remain quite rudimentary, the 
_ transverse processes are obliterated, and the 
-f head of the rib itself reduced to a ligamentous 
condition, the carapax being left sufficiently 
‘Strong without any necessity for the usual 
. _ abutments of the ribs on the vertebral column. 
___ In Birds, on the contrary, a precisely oppo- 
site arrangement is required in order to com- 
Py 
Tse i 
837 
bine strength and lightness in the construction 
of the framework\of their thorax, which must 
bear the strain ofthe strong muscles used in 
flight. The bifurcation of the commencement 
of the rib is here exaggerated to the utmost ; 
its strongly developed head is firmly articulated 
to the vertebral bodies, and by means of its 
tubercle it is additionally secured to the trans- 
verse processes of the dorsal vertebra, and 
moreover, besides the strong buttresses thus 
made to sustain the thorax, additional long 
splints of bone project backwards from the 
dorsal ribs much in the same manner as in 
Fishes, only here the superadded processes are 
prolonged until they overlap the rilb succeeding 
next behind, binding the whole together; and 
materially assisting to strengthen the thoracic , 
framework. 
But even in Birds, as in the Crocodile, the 
dorsal ribs are found developed from the ver- 
tebre anterior to as well as behind the proper 
thorax. 
In Mammals, the great portion of the chest 
consists of dorsal ribs, which are eked out in 
front by costal cartilages connecting them on 
each side to the sternum. Yet still the floating 
ribs behind the proper thorax are persistent, 
and in one rare instance, namely, the Sloth, 
they exist in front as well, appended to what 
else the anatomist would cal] cervical vertebra. 
We therefore see at once that the division of 
the spine into the different regions pointed out 
in the human skeleton is quite arbitrary, as the 
existence of ribs and the possession of a thorax 
are by no means necessarily linked together. 
A very singular illustration of the co-exist-" 
ence of thoracic and non-thoracic ribs is met 
with in Reptiles belonging to the remarkable 
genus Draco, in which, although the anterior 
ribs are completely developed so as to form a 
true chest, the six hinder pairs are converted to 
a totally different use, being prolonged laterally 
to a great extent, and covered with a duplica- 
ture of the integument so as to form an ample 
parachute, by the assistance of which these 
agile little lizards are in some degree supported 
in the air as they leap from branch to branch. 
The thoracic portion of the skeleton is only 
met with ina complete state in Birds and the 
higher Reptilia, the Saurians and Chelonians, 
in which races it constitutes a very elaborate 
framework composed of numerous elements, of 
which no traces are perceptible in the human 
subject or in the generality of Mammalia. In the 
Crocodile it is seen to be made up of the fol- 
lowing parts—1st, of a complete apparatus of 
dorsal ribs (fig. 482, 1), connected to the 
transverse processes and bodies of the dorsal 
vertebra ; 2ndly, of an equal number of sternal 
ribs (m_), interposed between the ends of the 
former and the sides of the sternum; and, 
3dly, of the sternum (n), forming the pectoral 
boundary of the chest. 
The sternum itself, although usually consi- 
dered by the human osteologist as being ex- 
tremely simple in its composition, is, when 
fully developed, made up of several distinct 
elements equalizing in importance any that 
assist in building up the skeleton. It is, how- 
* 
