TEXT-BOOK OF ANATOMY. 



INTRODUCTION. 



ANATOMY is a comprehensive term, which includes several closely related 

 branches of study. Primarily it is employed to indicate the study of the parts 

 which build up the body, and the relationship which these present to each, other. 

 But the structure of an individual is not the same at all stages of its life, for 

 many changes occur during the period of its existence. The ovum and the 

 spermatozoon, which are the starting-points of every individual, are very different 

 from the finished organism as represented by the adult, and the series of changes 

 through which the organism passes until its structure is perfected and full growth is 

 attained constitute the phenomena of development. The general term "development" 

 includes not only the various and striking structural changes which occur during 

 the intra-uterine life of the individual, to the study of which the term embryology 

 is more specially applied, but also many growth processes which occur after birth, 

 such as the later stages in the ossification and growth of the bones, the eruption of 

 the two series of teeth, the adjustment of the vascular system to its new require- 

 ments, etc. The actual observation of the processes by which the parts of the body 

 are gradually formed, and of the structural arrangements by means of which a 

 temporary connexion is established between the ovum and the mother, through 

 which an interchange of nutritive and other matters between the two takes place, 

 renders embryology one of the most interesting of all the departments of anatomy. 

 The term ontogeny also is used to denote the development of the individual. There 

 is, however, another form of development, slower, but just as certain in its pro- 

 cesses, which affects not only the individual, but all the members of the animal 

 group to which it belongs. The theory of descent or evolution leads us to 

 believe that between man of the present day and his remote ancestors there is a 

 wide structural gap, which, if the geological record were perfect, would be seen to 

 be completely occupied by long-lost intermediate forms. In the process of evolution, 

 therefore, structural changes have gradually taken place which have modified 

 the entire race. These evolutionary phases constitute the ancestral history or 

 phylogeny of the race. Ontogeny and phylogeny are intertwined in a re- 

 markable manner, and present certain extraordinary relationships. In other 

 words, the ancestral evolutionary development appears to be so stamped upon an 

 individual that it repeats certain of the phylogenetic stages with more or less clear- 

 ness during the process of its own individual development. Thus, at an early period 

 in the embryology of man evanescent gill-pouches appear which are comparable with 

 those of a fish, whilst a study of the development of his heart shows that it passes 

 through transitory structural conditions similar, in many respects, to the permanent 



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