Anatomy of the Rabbit. 



THE INTERPRETATION OF STRUCTURE. 



Gross structure is, in a sense, only the outward expression of the 

 finer microscopic structure underlying it, the latter being the true basis 

 of the body. This refers not so much to the individual features of 

 organs as to the relation existing between their appearance as gross 

 objects and their tissue composition. Since this relation is more fully 

 discussed below under the head of general anatomy, it need only be 

 mentioned here as an element in the interpretation of structure as 

 viewed from the gross standpoint. All structure, however, may be con- 

 sidered from two points of view — physiological, and morphological. The 

 former is more easily understood. All parts of the body are constructed on 

 a basis of use or function ; and although the various processes and activi- 

 ties of the body are more properly considered under the head of physio- 

 logy, a thorough conception of the anatomy of an organ is usually to be 

 gained only by a consideration of its particular role in the general 

 economy. 



The morphological aspect of structure concerns various features of 

 form and arrangement which, although they have been developed on a 

 basis of utility, cannot be explained directly on that basis, because the 

 factors controlling them lie outside of the body of the individual, and are 

 such as have operated only through a long series of gradually changing 

 conditions in the evolution of its type. As applied to a particular 

 animal, the morphological method consists in explaining its adult 

 structure by reference either to its embryonic development, or to the 

 equivalent conditions in lower existing, or perhaps fossil, forms. The 

 recognized principle of embryology is that known as the Law of Re- 

 capitulation. It is based on the general observation that the definitive 

 structure of an organism is attained through a series of embryonic stages, 

 in which it not only develops from a simple or ground type to a more 

 complex condition, but also reflects in passing the features of lower, and 

 presumably its own ancestral, forms. That of comparative anatomy 

 depends on the comparison of higher, specialized animals with lower, or 

 generalized ones, the latter being assumed, in one feature or another, to 

 have remained in a backward or primitive state of specialization, and 

 therefore to reflect in such features a low grade of structure of a kind 

 possessed by the ancestors of existing higher forms. These relations 

 . form a basis for the comparison of the embryonic development of or- 

 ganisms with the evolution or history of the groups which they represent, 

 the former being distinguished as ontogeny, the latter as phylogeny. 

 The' interpretation of the adult structure of an organism is a matter of 

 distinguishing its more general features from its more special ones, the 

 former being in all cases those to which the ontogenetic and phylogenetic 

 principles are especially applicable. 



All characters of animals have an evolutionary basis, the general 



