ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 17 



■with the sternum. These sternal ribs are homologons with the 

 costal cartilages of man. 



Sternum. — The sternum or breast bone of birds necessarily 

 attains an unusually large size in most species of the class, since 

 the muscles used in flight are mainly attached to it. In the adult 

 bird {^\. I. St. pi. III., figs. 1, 2, 3) the sternum consists of a 

 single complex bone, but in the foetus it is developed from at 

 least two and generally from five distinct centres of ossification. 



The sternum of an ordinary winged bird (pi. III., fig. 3 

 ■which represents the sternum of a fowl) consists of a median 

 ridge of bone (ca.) which is known as the carina,* and which 

 stands out at right angles from a curved bony shield which forms 

 the body of the sternum (figs. 1 and 2, st.) One or two pairs 

 of membraneous fontanellesf frequently remain uuossified 

 in the inferior and lateral portions of the sternum, and give 

 rise to as many holes or notches separating slender processes 

 in the dry skeleton. The median one of these processes (mx.) 

 forms the downward continuation of the carina, and as being 

 the homologue of the xiphisternum| of mammals is called 

 the middle xiphoid process ; the pair of processes (ix.) next to 

 the middle one are known as the internal xiphoid processes^ 

 while the outermost pair (ex.) are called external xiphoid pro- 

 cesses. In the angle situated between the carina and the body 

 of the sternum are placed the powerful pectoral muscles which 

 move the wing in flight. The superior angle of the sternum 

 is sometimes developed into a median process or rostrum^ 

 (fig. 3, r.) ; the angles on either side of this median process are 

 known as the costal processes (c. p.) to which some of the ribs 

 articulate, the rest articulating lower down (pi. I.) At the 

 superior or anterior border of the sternum of most birds there 

 are situated two grooves which receive the distal extremities 

 of the coracoids (pi. III., figs. 1 , 2, eor.) 



Having now become acquainted with the typical stei'num 

 of a bird it is necessary to say a few words as to some of the 

 modifications which this bone may undergo in the diflereut 

 groups. The two great divisions of living birds, called Ratitse 

 and Carinatge were formed by Professor Huxley on account of 

 the character of the sternum, and from which character the two 



* Carina, a keel. 



+ Fontaaelle, 'Bvench fontanelle, a little fountain; originally applied to the un ossi- 

 fied gap which exists in the skull of the human infant at the junction of the parietals 

 and frontals. Subsequently used in anatomy for any aperture in a bone which does 

 not serve for the transmission of either a nerve or vessel, and which in the livino' 

 animal is generally closed by membrane. 



X Xiphisternum, from %'0OS, a sword, and 'iTtpvov the breast bone ; applied to 

 the lower termination of the mammalian sternum which in man is pointed. 

 § Rostrum, a beak, used frequently in anatomy fgr a bony process. 



