FUNCTIONS OF ANIMAL ORGANISMS 495 



cellular network of fibers which brings about coordinated movement 

 among numbers of cilia or flagella. The reversal of ciliary movement is 

 not understood. The existence of cilia in the early stages of the larvae of 

 metazoans is of common occurrence and cilia are even retained as the 

 locomotor organs in the adults of ctenophores, turbellarians, nemertines, 

 and rotifers. In metazoans generally, however, since the increased size 

 of the body makes ameboid movement impossible and ciliary movement 

 ineffective, locomotion comes to be accomplished by movements of the 

 whole body or by means of appendages developed for the purpose. At 

 no time during life in nematodes and arthropods are locomotor cilia 

 developed. 



Muscular movement is the direct result of oxidation processes in the 

 muscles. The oxidation, which takes place in a muscle after it has acted, 

 is, however, a recovery process tending to build the organ up ready for the 

 next contraction. This process involves the synthesis from carbohy- 

 drates of a complex and unstable compound which when the muscle is 

 stimulated breaks down into simpler compounds. The result is to disturb 

 the equilibrium in the cell to such a degree that there is a flow of cyto- 

 plasm and the contraction results. The protein part of the cell is not 

 affected. 



The contraction of a striated muscle fiber results from the sending in 

 to the muscle of a number of stimuli close together. Such a contraction 

 is termed tetanic, and its continuation gives rise to a condition known 

 as tetanus. What is called tonic contraction and the condition already 

 referred to as tonus (Sees. 242 and 511) is most marked in nonstriated 

 muscles. No chemical changes take place in the muscle during tonus and 

 therefore such a condition can be maintained without using up the 

 resources of the cells. It is present particularly in the nonstriated muscle 

 cells in the walls of the blood vessels and the alimentary canal. Fatigue 

 of muscles is due in part to the accumulation of waste matters within the 

 cell. A distinct nervous element is also involved. 



There are in higher vertebrates both red and white striated muscles 

 which seem to differ in several respects. The white muscles are more 

 irritable and contract more quickly, while the red ones are less irritable, 

 show a greater degree of contraction in tetanus, and maintain contraction 

 longer. 



517. Nervous Activities. — The basis of all nervous activity is the 

 irritability and conductivity which characterize living matter as such. 

 As has been noted in connection with the ameba, stimuli are varied in 

 kind and the response due to each has received its own particular desig- 

 nation. The response may be positive or negative, maximal or minimal, 

 or it may be given to an optimum degree of a stimulus. Among the 

 Protozoa the functions of irritability and contractility are not separated, 

 and the same cell both receives and responds to a stimulus. This is true 



