254 VERTEBRATES. 



roughly to the perforation in the transverse process of 

 the cervical vertebra. Examine one of the ribs near the 

 middle of the series, and note its shape, position, and 

 range of movability. Note that it has a sternal as well 

 as a vertebral portion, and note that the sternal portion 

 is more or less cartilaginous. Note that the central part 

 of the sternum, to which the ribs attach at their distal 

 end, is transversely segmented. Observe that the fore- 

 most segment (the manubrium) is large, and is produced 

 anteriorly in a prominent keel. Observe that the hind- 

 most segment (the xiphisternum) is a slender rod of 

 bone, supporting a broad, flat cartilage. Which ribs are 

 shortest ? Which longest ? Which have no sternal por- 

 tion ? Which have but one articulation with their corre- 

 sponding vertebrae ? Which articulate partially with other 

 than their own vertebras ? 



Note the decrease in size of the vertebrae posterior to 

 the first sacral, and the gradual disappearance of all the 

 processes. 



II. Atlas and Axis. The first and second cervical 

 vertebrae show special adaptive modifications, and have 

 received special names. The first is the atlas. It bears 

 the skull upon two concave, articular surfaces, which 

 meet the occipital condyles of the skull. It has no 

 centrum ; or, rather, its centrum is probably the odontoid 

 process of the second vertebra. A transverse ligament 

 divides the very large perforation ; and the division of 

 it anterior to the ligament is occupied by this odontoid 

 process, around which the atlas rotates. The spinal cord 

 occupies the space posterior to the ligament. 



The second vertebra is the axis. It has a broad, flat 

 centrum, produced anteriorly into the conical odontoid 

 process. A suture between this process and the centrum 

 is readily seen in young rabbits, and indicates that the 

 process has become attached to the second vertebra, while 



