OSSEOUS SYSTEM. (COMP. ANAT.) 



837 



complicated by 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 vertebra; 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 vertebra? becomes progressivelychanged 

 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 (Jig. 432, o) being attached to all 

 the transverse processes of the cervical vertebrae. 

 These, instead of being piolonged 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 Chehmiaii 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 

 vertebrae are prodigiously developed and spread 

 out into broad flat osseous plales firmly con- 

 nected with each other and with the tubercles 

 of the ribs by means of their broad serrated 

 margins ; this being accomplished, the usual 

 attachments between the head of the rib and 

 the spine become unnecessary ; the bodies of 

 the vertebrae remain quite rudimentary, the 

 transverse processes are obliterated, and the 

 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- 



bine strength and lightness in the construction 

 of the framework of their thorax, which must 

 bear the strain of the 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 rib 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- 

 tebrae 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 call cervical vertebrae. 

 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 in a 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. 432, I), connected to the 

 transverse processes and bodies of the dorsal 

 vertebra?; 2ndly, of an equal number of sternal, 

 ribs (in), 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- 



