ANATOMY 15 



meditm 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 Ave 

 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 tracheal rings as 

 joining the bronchi, while proximally 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 oi' 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 vertebrae, 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 Avith that of a Mammal, Reptile, or Amphibian ; the atlas 

 or first vertebra of a Crow is completely homologous Avith the same 

 part of a Dog. On the other hand, the wing of a CroAv is only 

 incompletely homologous with the arm of Man ; nor is the two-toed 

 foot of the Ostrich completely homologous Avith the four-toed foot 

 of a Fowl, although the various bones w.hich compose the feet in 

 both are complete homologues. 



Homologous organs are consequently developed from the same 

 parts of the embryos of the creatures Avhich 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 folloAvs 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, Avith the present state of our 



