2 TEXT-BOOK OF ANATOMY. 
through transitory structural conditions in many respects similar to the permanent 
condition of the heart in certain of the lower animals. It is in connexion with 
this that the phrase has arisen that every animal in its individual development or 
ontogeny climbs up its own genealogical tree: a saying which, taking it even in 
the broadest sense, is only partially true. 
The higher conceptions of anatomy, which are obtained by taking a general 
survey of the structural aspects of the entire animal kingdom, constitute morphology. 
The morphologist investigates the laws of form and structure, and in his generalisa- 
tions he gives attention to detail only in so far as this is necessary for the proper 
establishment of his views. The knowledge of anatomy which is required by the 
student of medicine is different. It is essentially one of detail, and often details 
important from the practical and utilitarian points of view have little or no 
morphological value. This want of balance in the interest attached to anatomical 
facts, according to the aspect from which they are examined, so far from being 
unfortunate, affords the teacher the means of making the study of anatomy at once 
fascinating and attractive. Almost every fact which is brought under the notice 
of the student can be accompanied by a morphological or a practical application. 
This it is that lightens a study which, presented to the student of medicine in any 
other way, would be at once dry and tedious. 
Certain terms employed in morphology require early and definite explanation. 
These are homology, serial homology, and homoplasy. The same organ repeated in 
two different animals is said to present a case of homology. But this morphological 
identity between these two organs must be proved beyond dispute before the 
homology between them can be allowed. In deciding this identity the great and 
essential test is that the two organs in question should have a similar develop- 
mental origin. Thus the fore-limb of a quadruped is homologous with the upper 
limb of man; the puny collar-bone of a tiger, the fibrous thread which is the only 
representative of this bone in the horse, and the strongly marked clavicle of the 
ape or man, are all, strictly speaking, homologous with each other. Homologous 
orzans in different animals usually present a similar position and a_ similar 
structure, but not invariably so. It is not uncommon for a muscle to wander 
somewhat from its original position, and many cases could be quoted in which 
parts have become completely transformed in structure, either from disuse or for 
the purpose of meeting some special demand in the animal economy. In the study 
of the muscles and ligaments instances of this will be brought under the notice of 
the reader. Identity or correspondence in the function performed by two organs 
in two different animals is not taken into consideration in deciding questions of 
homology. The gills of a fish and the lungs of a higher vertebrate perform very 
much the same physiological office, and yet they are not homologous. The term 
analogy is often used to express functional correspondence of this kind. Often 
organs which perform totally different functions are yet perfectly homologous. 
Thus the wing of a bat or the wing of a bird, both of which are subservient to 
flight, are homologous with the upper limb of man, the office of which is the 
different one of prehension. 
In the construction of vertebrates and certain other animal groups a series of 
similar parts are repeated along a longitudinal axis, one after the other. Thus the 
series of vertebrae which build up the backbone, the series of ribs which gird round 
either side of the chest, the series of intercostal muscles which fill up the intervals 
between the ribs, the series of nerves which arise from the brain and spinal cord, 
are all examples of this. An animal exhibiting such a condition of parts is said to 
present the segmental type of organisation, and in the early stages of development 
this segmentation is much more strongly marked, and is to be seen in parts which 
