GENERAL ORGANOLOGY 89 



absent; thus the digestive tract of coelenterates is only an epithelium; 

 their nervous system consists merely of a plexus of nerve-fibres and 

 ganglion-cells. 



Effect of Use and Disuse. It is of the greatest importance for the 

 permanency of an organ that it be constantly in function. Living sub- 

 stance is distinguished from the non-living by the fact that, if it be de- 

 stroyed by use, it is immediately replaced, often by more than sufficient to 

 make good the loss. Functioning tissues and organs under favorable 

 conditions increase in volume; on the other hand, functionless parts 

 undergo a gradual decrease, which finally leads to their disappearance. 

 To the extent that functioning tissues grow at the expense of those not 

 used there can be a 'struggle of the parts within the organism' (Roux). 

 There is also the same struggle between the structural elements in one and 

 the same tissue as is well shown in certain bones like the femur and tibia. 

 These retain only those bony parts, the outer tube and the bony bars at 

 the ends, which are necessary to support the body, all other parts being 

 absorbed. These bony bars are in the position which can be mathe- 

 matically demonstrated to be necessary to meet the strains. If the line of 

 strain be altered, as when a leg is broken and badly set, the bars are altered 

 in position to meet the new strains. 



Change of Function of Organs. The two factors mentioned, that 

 the permanence of the tissues depends upon continued use, and that 

 usually several tissues enter into the structure of an organ, are important 

 for the understanding of the principle of change of function which plays 

 a prominent role in the metamorphosis of animal form. It may happen 

 that an organ is brought under changed conditions and no longer has an 

 opportunity to function as before. In that case the functioning tissue, 

 from lack of use, gradually degenerates, but the organ may persist by 

 means of its accessory tissues if the new conditions make it possible for 

 one of them to attain to functional activity, and to give the organ a new 

 physiological character. 



A muscle, for example, may become functionless from many causes. 

 Should the muscle-tissue disappear there are still left the accessory tissues, 

 particularly connective tissue permeated by blood-vessels; this may remain 

 intact and form a protecting band, a tendon, or fascia. We have then, 

 morphologically, the same organ, changed in its physiological character; 

 the muscle has undergone a change of function, and has become a liga- 

 mentous band. The visceral arches of fishes primarily are supports for 

 the gills; if now by the acquirement of terrestrial habits the gills be lost, 

 the visceral arches become functionless and correspondingly undergo 

 a partial degeneration; but a part persists by assuming a new function, 



