MULLER] XHE AIR-SACS OF THE PIGEON 391 



tion of the foramen is constant for each bone ; its size and form are, 

 however, variable. Strasser (1877, P- l 9 2 ) was tne first to consider 

 the causes of that constancy and this variability. In mechanically 

 unimportant places the osteoclasts destroy the bony substance and 

 the osteoblasts do not produce any. Thus foramina are formed 

 through which the outgrowths of the air-sacs enter. The pneumatic 

 foramen is accordingly always found where the pressure, traction, 

 and torsion are least, a place determined under normal conditions 

 by the mechanical forces acting on each bone. The boundary of this 

 area of least stress, where the foramen is formed, is naturally not 

 sharply defined, and subsequently its margins are often partly covered 

 with osseous substance. These factors determine the constancy of 

 the location of these foramina and the variability of their shape and 

 size. The foramen is either simply circular or oval in form, some- 

 times, however, through subsequent deposit of bone, divided into 

 several smaller apertures and sieve-like. In the sternum there is 

 always a large foramen at the bottom of the concavity of its upper 

 side. Usually there are also other foramina on its upper surface on 

 either side, less frequently on one side only, which form a row parallel 

 to its lateral margin. The size of these foramina is very variable. 

 Sometimes a third, median, row of foramina is found on the upper 

 surface of the sternum. This shows that the occurrence of these 

 rows of foramina is very variable, whilst the position where they 

 occur when they are formed is always the same. These parts of the 

 bony wall, being unimportant mechanically, are very thin when no 

 foramina are present. 



The principal pneumatic foramen of the scapula lies near its artic- 

 ular end ; that of the coracoid on the inner surface of the canalis 

 triosseus ; that of the clavicle at the broad, spoon-shaped, scapular 

 end ; that of the humerus at the bottom of the deep depression on 

 the anterior side of its basal portion ; those of the ribs lie on their 

 concave surfaces near the joints ; those of the sterno-costalia on 

 their concave surfaces near the sternal border ; those of the pelvic 

 bones on their ventral surface, usually above the kidneys ; those of 

 the cervical vertebras on the surfaces turned towards the canalis 

 intertransversarius and at other places, for example, on the distal 

 end of the atlas ; and those of the dorsal vertebrae in various places. 



The Pneumaticity of Bones in Different Birds 



The foramen is formed, and the air-sac enters it at a comparatively 

 late period, when the development of the bone is already quite com- 

 plete, so that the bones of very young birds are not pneumatic. In 



