2 COMPARATIVE ANATOMY 



and 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 organisin, and the 

 combination of these to form tissues. Various combinations of 

 the tissues giye 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, and the different tissues may be divided 

 into four principal groups : — 



1. Epithelium, and its derivative, glandular tissue. 



2. Supporting-tissue (connective-tissue, cartilage, bone). 



3. Muscular tissue. 



4. Nervous tissue. 



In accordance with the functions they perform, epithelium and support- 

 ing-tissue may be described as passice, and muscular and ner\-ous tissue as 

 ri.cti'ce. 



By an organ we understand an apparatus constructed to 

 perform 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 defined above are united 

 together as Morphology, as opposed to Physiology which con- 

 cerns the functions of organs, apart from their morphological rela- 

 tions. The results obtained from these two fields of study help to 

 complete one another, and thus to throw light on the organisation 

 of animals in general — that is, on Zoology in its widest sense. 



II. DEVELOPMENT AND STRUCTURAL PLAN OF THE 

 VERTEBEATE 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 cf a rounded vesicle (Fig. 1), in the interior 

 of which the following parts can be distinguished : — the vitcllus.. 



