THE SKELETON 425 



ribs (cervical and thoracic) arise a short distance lateral to the 

 vertebral centers, with which they are connected only by the 

 intervening membrane, which forms the vertebro-costal liga- 

 ments. Chondrification then proceeds distally. 



The cervical ribs chondrify from a single center. The thoracic 

 ribs have two centers of chondrification; a proximal one, corre- 

 sponding to the vertebral division of the rib, and a distal one 

 corresponding to the sternal division. The lumbar and sacral 

 membranous costal processes do not chondrify separately from 

 the vertebral bodies; if they persist at all, therefore, they appear 

 as processes of the vertebra, and are not considered further. 



In the fowl the atlas does not bear ribs, and in the embryo the primary 

 costal processes of this vertebra do not chondrify. The second to the 

 fourteenth vertebrae bear short ribs, with capitulum and tuberculum 

 bounding the vertebrarterial canal. The fourteenth is the shortest of 

 the cervical series. The fifteenth and sixteenth vertebrae bear relatively 

 long ribs, but, as these do not reach the sternum, they are classed as 

 cervical. The entire embryonic history, however, puts them in the 

 same class as the following sternal ribs; on an embryological basis they 

 should be classed as incomplete thoracic ribs. They possess no sternal 

 division, but the posterior one has an uncinate process like the true tho- 

 racal ribs. The following five pairs of ribs (vertebras 17-21) possess 

 vertebral and sternal portions, but the last one fails to reach the sternal 

 rib in front of it. 



The vertebral and sternal portions of the true thoracal ribs 

 meet at about a right angle in a membranous joint. This bend 

 is indicated in the membranous stage of the ribs. 



The membranous ribs growing downwards and backwards 

 in the wall of the thorax make a sudden bend forward, and their 

 distal extremities fuse (seven and eight days) in a common mem- 

 branous expansion (primordium of the sternum), which, however, 

 is separated from the corresponding expansion of the opposite 

 side by a considerable area of the body-wall. 



The vertebral and sternal portions of the ribs ossify separately; 

 the ossification of the ribs is exclusively perichondral up to at 

 least the sixteenth day (cf. Fig. 242). 



The uncinate processes were not formed in any of the embryos 

 studied. Apparently they arise as separate membranous ossi- 

 fications after hatching. 



The sternum takes its origin from a pair of membranous expan- 



