430 ORGAN SYSTEMS OF MAN 



blood or lymph on one side and discharge dramatic. Such fish, when moved experi- 

 it on the other into a lumen or tube that mentally from an aquarium with a black 

 conducts the product to its proper place, bottom, over which they become very dark, 

 either to the outside of the body, in tlie case to one of white sand, will shortly become a 

 of sweat glands, or into the digestive tract, very light color. Furthermore, some, like 

 in the case of digestive glands ( Fig. 16-25 ) . the flounder, will actually produce the mot- 

 In such glands as sweat and tear glands no tied effect of colored stones on the ocean 

 special substances are synthesized; their floor. Lizards will change from the color of 

 secretion is merely extracted directly from the bark on the tree trunk to the intense 

 the blood. However, in the case of many green of the leaf. The protective value of 

 other glands, the digestive and endocrine such a mechanism is obvious, 

 glands for example, while the raw materials This color change is accomplished by 

 are extracted from the blood stream, the the movement of pigment either in the 

 final product is synthesized within the cell effector end organ in the skin or closely 

 itself. Just how this is accomphshed is un- associated with it. In vertebrates, the pig- 

 known, ment is confined to single cells which are 



Glands respond to stimuli from the nerv- scattered throughout the skin of the animal, 

 ous system and most of them secrete only When light falls on tlie eyes of the fish, 

 when stimulated. Some endocrine glands, stimuli are sent to the chromatophores 

 however, apparently secrete continuously, which adjust the amount of pigment that 

 although the rate may be affected by nerv- spreads out on the surface to obtain just 

 ous excitation or by hormones from other the right shade of color to match the back- 

 endocrine glands. There is considerable ground. Blind fish remain one color no 

 energy utilized during glandular activity, matter what the background. On a light 

 Thus the sweat glands produce a fluid with background, the normal response of each 

 a salt content considerably higher than that chromatophore is to concentrate the pig- 

 of the blood, and energy is required to in- ment granules of the cell into compact "pin 

 crease this concentration just as it would points." On a dark background the same 

 be if heat were used to evaporate an equal pigment granules spread out at the surface, 

 amount of dilute salt water to a similar con- darkening the entire area, 

 centration (Fig. 16-25). Careful measure- Although most of the pigment cells in 

 ments of gland cells demonstrate that some animals are primarily under the influence 

 respire at a higher rate than any other cells of the nervous system, some, such as those 

 of the body, even those of the heart. of Crustacea (Fig. 16-26) and Amphibia 



(Fig. 16-27), are controlled by hormones, 



Chromatophores although the initial stimulus is via the eyes. 



Although color change is not found in Pieces of frog skin, for example, can be 

 human beings other than the gradual tan- stimulated to change color simply by dip- 

 ning of the skin as a result of exposure to ping them in solutions containing specific 

 sunlight, many lower animals are equipped hormones. However, under normal condi- 

 with very efficient and often spectacular tions, stimuli from the eyes excite the en- 

 color-changing apparatus. In some animals docrine glands whose secretion causes the 

 such as the squid (Fig. 12-17) the variety chromatophores to respond. It resembles a 

 and rapidity of change in color is almost chain reaction, 

 fantastic. It can change from pearly white 

 to intense black almost instantaneously. '° "*"' 



Others such as fish and reptiles change This is often spoken of as "cold light" 



more slowly but the final product is equally because very little heat is emitted and for 



