Mammalia. 161 



row of neural spines forming the ridge of the backbone. 

 Extending outward on each side are the two transverse processes. 

 Near the end, on the dorsal surface, are the two smooth facets, 

 where the vertebra joined the vertebrae before and behind it; 

 these are the articulating processes. Do all the vertebras have 

 the same number of the processes ? Where the same- number 

 is present are they alike ? What range of motion is allowed 

 between two vertebrae? Is this equal in different parts of the 

 spinal column? 



6. Study carefully the first and second vertebras. The first is 

 the atlas. Has it a body ? Note how the two hollowed facets 

 on each side of the hole fit the two occipital condyles of the base 

 of the skull. How is the nodding motion of the head accom- 

 plished ? The second vertebra is the axis. Projecting forward 

 from it is a peg which extends into the opening in the atlas ; 

 this peg is the odontoid process. Observe that when the head 

 turns from side to side it is by turning on this axis. 



7. Examine one of the middle ribs. Find that it joins the 

 backbone in two places, by its head on the side of the vertebra, 

 and by a little projection called the tubercle, with the tip of a 

 transverse process. What range of motion has a rib? Note 

 that at the ventral end the rib is cartilaginous. What is the 

 advantage of this fact ? Compare the series of ribs. Examine 

 the breastbone. Is it of one piece ? Is there any cartilage in 

 it ? What are the uses of the ribs ? Is there a collar bone ? 



8. In the fore limb look at the shoulder blade or scapula. 

 How does it make up in strength for its thinness ? Why should 

 it be flat ? Note the shallow cavity by which it articulates with 

 the bone of the upper arm, the humerus. This is a ball-and- 

 socket joint. Look closely for a slender collar bone, or clavicle. 

 In the forearm the longer bone is the ulna; the other is the 

 radius. Do they rotate on each other as in our forearms ? 

 Compare them also with the corresponding bones of a cat. Do 

 they need the same freedom of motion as in a cat, squirrel, ape, 

 or man ? The bones of the wrist are the carpal bones. The 



