ANATOMY 15 
median or inner, lateral or outer. Lastly, since it is not always 
obvious to which axis or plane a given organ is to be referred, its 
parts can be described with reference to its neighbours. Hence we 
speak of the tibial and fibular, radial and ulnar side of the bones 
and other parts of the extremities ; the fourth toe is on the fibular, 
outer, or lateral side of the foot, the first, which is ordinarily the 
hind toe, on the tibial, inner, and posterior side. 
The basal part of an organ is generally also its proximal part or 
root, while the apex corresponds with its free or distal end, the 
latter being the portion most removed or distant from the region 
whence it grew. Thus we speak of the distal traclieal rings as 
joining the bronchi, while proaimally the trachea is attached to the 
larynx. 
In comparing the various parts of one animal with each other, 
or with those of another animal, we call the organs which are 
morphologically or structurally similar homologous, the parts which 
physiologically or functionally correspond are analogous. When the 
comparison is restricted to one individual, the homologies are general. 
The different vertebra, or the ribs, or the anterior and posterior 
extremities of any particular Bird are serially homologous or homo- 
dynamous organs, because they are to a certain extent repetitions 
of each other, although not necessarily exactly alike. If the 
comparison refers to similar organs in various individuals, no 
matter if these belong to the same species, genus, family, or class, 
the homologies are special, and these again may be complete or incom- 
plete. For instance, the humerus of a Bird is completely homo- 
logous with that of a Mammal, Reptile, or Amphibian ; the atlas 
or first vertebra of a Crow is completely homologous with the same 
part of a Dog. On the other hand, the wing of a Crow is only 
incompletely homologous with the arm of Man; nor is the two-toed 
foot of the Ostrich completely homologous with the four-toed foot 
of a Fowl, although the various bones which compose the feet in 
both are complete homologues. 
Homologous organs are consequently developed from the same 
parts of the embryos of the creatures which are under comparison. 
Hence the number of existing homologies in given animals indicates 
their further or closer relationship, and is used for assigning these 
animals to their places in the system. It follows from this con- 
sideration, that the animal’s place in the system depends greatly, or 
entirely, upon the characters or organs selected for this purpose. 
Unless all the organs and all their characters are carefully considered, 
not only in the few Birds which happen to occupy our attention at 
the time, but also in Birds of as many different groups as it is pos- 
sible to examine, our attempts to produce a classification of Birds 
must invariably end in the production of arbitrary “keys.” It is 
extremely difficult, often hopeless, with the present state of our 
