88 GENERAL PRINCIPLES OF ZOOLOGY 



1 6. According to the nature of the muscle-substance are distinguished 

 smooth and cross-striated muscle-fibres. 



17. According to the character and origin of the cells (muscle-cor- 

 puscles) the muscles are divided into epithelial (epithelial muscle-cells, 

 primary bundles) and connective-tissue muscle-cells (contractile fibre- 

 cells). ' 



1 8. The physiological distinction of nervous tissue rests upon the 

 transmission of sensory stimuli and voluntary impulses, and upon the 

 co-ordination of these into unified psychic activity. 



19. The conduction takes place by means of nerve-fibres (non-medul- 

 lated 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 amoeboid cells (white blood-corpuscles, 

 leucocytes), or in addition to these also red blood-corpuscles. 



21. Red blood-corpuscles occur in vertebrates and cause the redness 

 of the blood; they are absent in most invertebrates. 



22. When invertebrates have colored blood (red, yellow, green), this 

 is usually due to the color of the blood-plasma. 



23. The red blood-corpuscles are non-nucleated in mammals, nucle- 

 ated in all the other vertebrates. 



III. THE COMBINATION OF TISSUES INTO ORGANS. 



An Organ Defined. Organs are formed from the tissues. An organ 

 is a tissue complex, marked off from the other tissues, which lias 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 as a connected whole from its 

 environment and can still accomplish a definite movement. 



Principal and Accessory Tissues. In each organ there is a tissue 

 which determines- its function, and therefore its physiological character. 

 This is called the principal tissue, for there may be accessory tissues 

 present, which merely support or render possible the action of the prin- 

 cipal tissue. In the muscle of the vertebrates we find, besides the muscle- 

 fibres, connective tissue which unites the bundles of muscle; blood-vessels 

 which provide nourishment; finally, nerves by which the muscles are 

 aroused to action. In the human liver also, besides the functionally 

 most important part (the liver-cells), blood-vessels, nervous and connective 

 tissues occur. These ^accessory tissues are usually found only in the 

 highly developed organs; in the case of the lower animals they may be 



