2 TEXT-BOOK OF ANATOMY. 



conditions 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 broader 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 instructive. Almost every fact which is brought under the notice 

 of the student can be accompanied by a morphological or a practical application. 

 These possibilities of application lighten 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 the morphological 

 identity between the two organs must be proved beyond dispute before the 

 homology between them can be allowed. In deciding the 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 one another. Homologous 

 organs in different animals usually occupy a similar position and possess 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. 



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. 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. 



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 vertebrse which build up the backbone, the series of ribs which gird round 

 each 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 medulla, 

 are all examples of this. An animal exhibiting such a condition of parts is said to 

 present the segmental type of organisation. In the early stages of development 

 this segmentation is much more strongly marked, and is to be seen in parts which 



