2 COMPARATIVE ANATOMY 



rudimentary or vestigial organs and parts in the adult organism 

 which would otherwise remain totally inexplicable. 



Histology is a subdivision of anatomy which concerns the 

 structural elements the building-stones of the organism, and the 

 combination of these to form tissues. Various combinations of 

 the tissues give rise to organs, and the organs, again, combine to 

 form systems of organs. 



The structural elements consist primarily of cells and second- 

 arily of cells and fibres often enclosed in an intercellular substance 

 or matrix', and the different tissues may be divided into four 

 principal groups : 



1. Epithelium, and its derivative, glandular tissue. 



2. Supporting tissue (connective tissue, adipose tissue, cartilage, 

 bone). 



3. Muscular tissue. 



4. Nervous tissue. 



In accordance with the functions they perform, epithelium and 

 supporting tissue may be described as passive, and muscular and 

 nervous tissue as active. 



By an organ we understand an apparatus which performs a 

 definite function : as, for instance, the liver, which secretes bile ; 

 the gills and lungs, in which an exchange of gases is effected with 

 the surrounding medium ; and the heart, which pumps blood 

 through the body. 



The organ-systems, which will be treated of in order in this 

 book, are as follows : 1. The outer covering of the body, or inte- 

 gument ; 2. The skeleton; 3. The muscles, together with electric 

 organs ; 4. The nervous system and sense-organs ; 5. The organs of 

 nutrition, respiration, circulation, excretion, and reproduction. 



The closely-allied branches of science denned above are in- 

 cluded under the term Morphology, as opposed to Physiology 

 which concerns the functions of organs, apart from their morpho- 

 logical relations. The combined results obtained from these two 

 fields of study throw light on the organisation of animals in 

 general that is, on Zoology in its widest sense. 



II. DEVELOPMENT AND STRUCTURAL PLAN OF THE 

 VERTEBRATE BODY. 



The structural elements described in the preceding section as 

 the building-stones of the organism, i.e. the cells, all arise from a 

 single primitive cell, the egg-cell or ovum. This forms the 

 starting-point for the entire animal-body, and a general account of 

 its structure and subsequent development must therefore be given 

 here. 



The ovum consists of a rounded, nucleated, protoplasmic body 



