20 



ANATOINIY OF VERTEBRATES. 



more of the last pairs of thoracic ribs are iinconuected with each 

 other, in the skeleton ; such sternal ribs resembling the abdominal 

 hasmapophyses in Saurians, or the ' intersections,' ossified, of the 

 rectus abdominis muscle in Mammals. 



The modifications of the sternum in Birds relate to their 

 faculty of flight ; more directly, to the adequate origin of the 

 muscles acting upon the pectoral limb, less directly to the me- 

 chanism of respiration needed by the conditions of the lungs ; also, 

 in Perchers, to sustaining the body in sleep. 



The sternum of the Bird is the bony ventral wall of the trunk, 

 fig. 18, 60, s, r; it is not, however, the 

 homologue of the plastron of the Tortoise, 

 fig. 53, p. 63, vol. i., but of the series of 

 haemal spines forming the episternum and 

 sternum of the Crocodile (fig. 56, p. 68, 

 ib.) ; it is developed, in most Birds, from 

 one pair of ossific centres, wdiich, coalescing 

 in the midline, usually consolidate the car- 

 tilaginous basis of the keel by continuous 

 ossification therein. 



The chief parts to be noted in the Bird's 

 sternum are the ' body,' fig. 15, a, mth its 

 notches or holes, f, f; the 'keel,' fig. 18, 

 s; the 'costal processes,' fig. 15, d; the 

 ' costal borders,' Avith their articular sur- 

 faces, fig. 15, <? ; the ' coracoid grooves,' figs. 

 15 and 16, b; and the ' manubrium,' fig. 15, 

 e, 59. 



The body may be almost flat, as in 

 Apteryx and Dlnornis ; or very concave, 

 the sides being bent upw^ard at an acute angle, as in Aqnila ; 

 it is commonly less concave toward the trunk. It varies 

 greatly in the proportions of length and breadth : the latter 

 dimension is in excess in Apteryx and Dinoniis ; the breadth 

 nearly equals the length in other Struthionidce, the Albatross, 

 and the Pelican. The length progressively gains in other birds, 

 until it becomes four times the breadth of the sternum in Tina- 

 mus, fig. 14, B. Extreme length is associated wdth ordinary 

 l)readth in the sternum of the Anserines, Auks, Guillemots, 

 many Waders, diurnal Baptores, and some Volitores, reaching to 

 the pelvis, and occasionally to the pubic bones, fig. 18, s,p', and 

 requiring removal for exposure of the abdominal cavity. 



Examples of an ' entire ' sternal body, i. e., neither notched nor 



Cantorial sternum, Sbrike. 



