15^ 



CHORDATE ANATOMY 



and a less dense anterior half. As the definitive vertebrae are formed, 

 the posterior half of each vertebral anlage fuses with the anterior half 

 of the following one. By this process the definitive vertebrae come to lie 

 intersegmentally, alternating with the myotomes. The result is obvi- 

 ously adaptive, since only by this arrangement could each myotome 

 become connected with two vertebrae and with two successive ribs. 



The Ribs. Man has twelve pairs of ribs, which form a basket sur- 

 rounding the thoracic cavity. Each rib is a curved flat bone ending 

 ventrally in a costal cartilage. By means of these costal cartilages the 

 first seven pairs connect directly with the sternum and are therefore 



EPIDERMIS 



DERMATOME 



MYOTOME 



-SCLEROTOME 



NOTOCHORD 



-SCLEROTOME 



°o"0 



MYOTOME 

 DERMATOME 

 .Ssils^^^^^s^j^SsSCsSs^i^^SjS^is^^s^^s^tfR^^J^EPIDERMIS 



Fig. 142. — A diagram showing the relations of myotome and sclerotome as seen in a 

 horizontal section of a vertebrate embryo. The upper half of the figure shows the 

 relations in an earlier .stage of development, while the lower half represents a later stage. 

 The posterior half of each sclerotome unites with the anterior half of the following 

 sclerotome to form a centrum which thus alternates in position with the adjacent 

 myotome. Thus each myotome becomes attached to two vertebrae. 



called sternal or true ribs while the five remaining pairs are distinguished 

 as "false" ribs. The last two pairs, the eleventh and twelfth, do not 

 connect with others, and are known as floating ribs. Each rib has a head 

 or capitulum, which articulates with the vertebral centrum, and a tuber- 

 culum, which articulates with the transverse process. As the rib basket 

 rises and falls in breathing, each rib rotates on an a.xis running through 

 the tuberculum and the capitulum. 



Each rib has a costal groove extending along its lower or posterior 

 border. To the ridges which border this groove are attached the external 

 and internal intercostal muscles. 



The Development of Ribs. Ribs develop in the embryo as costal 

 processes of the vertebrae in the intermuscular septa or myocommata. 

 Primarily, the cartilaginous anlagen of the ribs are continuous with the 



