MAMMALIA AND BIRDS. 313 



we have the fore-leg referred to the muscles of the 

 shoulder and the spinous processes by which they are 

 supported in the mammalia ; and though both are 

 so placed that neither the one nor the other directly 

 disturbs the vertebral column, so as to derange its 

 important contents, yet they are very beautifully 

 arranged so as to suit the different systematic habits 

 of the animals. In the mammalia, an action of the 

 thorax in heaving and falling is required, in order to 

 carry on the process of breathing by means of lungs 

 and a moveable diaphragm exclusively; and there- 

 fore neither the weight of the anterior part of the 

 body, nor the motion of the fore-legs, could be, in 

 them, supported from the sternum, without confining 

 and disturbing that action. Thus the mammalia, 

 when they bear the weight on the fore-legs, have the 

 thorax free as it were, and suspended upon these fore- 

 legs as props, the muscles and other soft parts, which 

 attach the scapular bone to the body, affording an 

 elastic support, similar but superior to that of the 

 springs upon which the body of a carriage is hung. 



Birds, on the contrary, never have any support, 

 except by the feet, upon substances which are not in 

 themselves elastic, as the air or the water, and thus 

 the anterior part of their bodies is carried on the 

 sternum, as a basket or boat, that bone being always 

 more produced and developed in proportion as the 

 weight of the body is borne more upon it. If the 

 bearing is chiefly upon the wing, then the strength of 

 the sternum is concentrated forwards ; but if upon 

 the under surface of the body, as in the swimming 

 birds, then the sternum is produced backwards, or has 

 more of a boat shape. In birds that fly little, the 

 sternum is weak ; and in those which cannot fly it is 

 short, that it mav be out of the way of the more 



