1172 The “Collar Bone” in the Mammalia. [{December, 
‘THE SIGNIFICANCE OF THE “COLLAR BONE” IN . 
THE MAMMALIA. 
BY SPENCER TROTTER, M.D. 
HEN running through a series of forms in animal life a 
structure is found fully developed in some and in others 
rudimentary or entirely wanting, we are led to consider the 
causes acting upon the structure through the life of the animal 
which has brought about the condition of development or non- 
‘development present. Every fully developed tissue in an organ- 
ism is needed or it would not be there; and just so soon as by 
increasing change in life and habits, it becomes a factor of less 
and less importance to the animal; it fails more and more to 
attain its former standard of development, and in time falls back 
to the primitive condition from which it arose and finally dis- 
appears. 
The “collar bone,” or clavicle is an unstable factor in the 
shoulder girdle; by this I mean an element not always found 
present throughout vertebrates, and its presence, absence or rudi- 
mentary condition, is in relation to the life of the animal. 
In this article it is my intention to note, principally among the 
mammals, the relation which the clavicle bears to the various 
modes of life, but before taking up the consideration of individ- 
ual forms, reviewing briefly the general anatomy of the part. 
The shoulder girdle consists first of the scapula, or “ shoulder 
blade,” a more or less irregular plate of bone preformed in car- 
tilage and ossifying from two centers, the dorsal or scapular, the 
ventral or coracoid, in position against the anterior thoracic ribs, 
its long axis varying in inclination. 
Its ventral end terminates in the “ glenoid cavity,” a ridge and 
process, spinous and acromion, on its outer surface are more of 
less developed in different forms, and in all mammals above 
_ the Ornithodelphia the coracoid is reduced to a mere process. 
Second, the clavicle, when present, preformed in fibrous tissue 
extends as a bar of bone from the acromion process above the 
-~ glenoid cavity to the manubrium sterni, forming a strong support 
to the girdle and an extended surface for ligamentous and mus- 
cular attachments. Mechanically considered the shoulder girdle 
(by using the term “ girdle” both sides of the body are implied) 
nothing more than the fulcrum of which the fore limb is the 
of of the aed kind, and its specialization is in direct relation 
