THE SKELETON. 337 



The Cavities of the Skeleton. Examine the cavity o/~ the skull. 

 If the class has not a skull which has been sawed across, look into the 

 skull cavity through the hole where the spinal cord joined the brain. 



Observe the conical shape of the thorax. In the entire body the 

 bones and muscles about the shoulders usually make a reversed cone of 

 the upper part of the trunk. 



Observe that the ribs are connected with the breastbone by carti- 

 lages. 



The upper limbs are articulated with the body only where the inner 

 ends of the collar bones join the breastbone. 



Pronation and Supination. Rest the forearm on the table with the 

 palm up ; keeping the elbow fixed, turn the hand over. Turning the 

 palm up is called supination ; turning it down is pronation. Perform 

 this experiment with the articulated skeleton. 



The Skeleton of a Cat or Rabbit. Examine the skeleton of a cat 

 or rabbit for the sake of comparison. Note especially the skull and 

 spinal column, so that you will know better what to do when dissecting 

 the brain and spinal cord in one of these animals. 



The Weight of Bones. The bones make about one sixth 

 of the weight of the living body. When dried they may 

 lose half of their weight. 



Microscopic Structure of Bone, i . Examine with a hand lens. 

 Hold a mounted cross-section of bone up to the light and examine with 

 a hand lens. The solid part of the bone will be seen to be pierced by 

 many small holes (or if the holes are filled they will appear as black 

 spots). These are the cross-sections of the haversian canals, through 

 which run the blood tubes, mainly lengthwise through the bone. 



2. Examine with the Low Power of a Compound Microscope. 

 Examine the section under the microscope, using a half-inch objective. 

 The bony matter will now be seen to be arranged in circles, lamellae, 

 around the haversian canals, somewhat like the rings seen on the end 

 of a log. 



Between the rings are circles of elongated dark dots. These are 

 lacunae, cavities in which lay the live-bone corpuscles which built up 

 the bone. The bone was, at first, cartilage. Later, mineral matter 

 was deposited, forming true bone. 



