Il8 GENERAL PRINCIPLES OF ZOOLOGY. 



epithelial (epithelial muscle-cells, primary bundles) and 

 connective-tissue muscle-cells (contractile fibre-cells). 



1 8. The physiological character of nervous tissue 

 rests upon the transmission of sensory stimuli and volun- 

 tary impulses, and upon the co-ordination of these into uni- 

 fied psychic activity. 



19. The conduction takes place by means of nerve- 

 fibres (nonmedullated and medullated fibrils and bundles of 

 fibrils) ; the co-ordination of stimuli by' means of ganglion- 

 cells (bipolar, multipolar ganglion-cells). 



20. Blood and lymph are proteid-containing fluids; 

 rarely without cells, they may contain only colorless amoe- 

 boid cells (white blood-corpuscles, leucocytes), or in addition 

 to these also red blood-corpuscles. 



21. Red blood-corpuscles occur, in the main, only in 

 vertebrates and cause the redness of the blood ; they are 

 absent in most invertebrated animals. 



22. If invertebrated animals have colored blood (red, 

 yellow), the cause for this is usually to be found in the 

 blood-plasma. 



23. The red blood-corpuscles are nonnucleated in mam- 

 mals, nucleated in all the other vertebrates. 



III. THE COMBINATION OF TISSUES INTO ORGANS. 



An Organ Denned. From the tissues organs are 

 built up. An organ may be called a complex of tissues 

 which is marked off from the other tissues and has taken on 

 a definite form for carrying on a special function. Thus a 

 single muscle is an organ which consists of a certain 

 amount of muscular tissue ; with scalpel and scissors it can 

 be removed from its environment and still accomplish a 

 definite movement. 



Principal and Accessory Tissues. In each organ 

 there is a tissue which renders possible the function of the 

 organ, and therefore chiefly determines its physiological 

 character. This may be called the principal tissue, for 

 there may be other tissues present, the accessory tissues, 



