HISTOLOGY 151 



the stimulus produced by the appearance of the insect. Within 

 these retinal cells nervous impulses are set up and pass along 

 nerve-cell processes toward the brain (c/. Fig. 67, p. 123). In the 

 visual centers of the brain, the impulses are transmitted to other 

 cells of the nervous sj^stem, and eventuall}^ go out along nerve 

 fibers leading to the muscle cells of the frog's tongue. The effect 

 produced is the contraction of certain muscle cells, resulting in the 

 movement of the tongue for the capture of the prey. The contact 

 of the insect with the lining of the frog's mouth causes the closing 

 of the mouth, and this in turn stimulates the act of swallowing. 

 These activities, of course, result from muscle-cell reactions. In 

 the stomach and intestine, the insect is digested by the juices 

 secreted by gland cells occurring in the wall of the tract and in 

 the pancreas and liver. These juices are secreted at the proper 

 time because of the coordinating mechanism of nerve cells and 

 their processes, or because of the production of endocrines by cer- 

 tain cells and their effect upon other cells {cf. p. 97). After 

 digestion and absorption have occurred, the simple foods are car- 

 ried in the blood plasma to capillary networks where they pass out 

 through the endothelial walls of the capillaries to the many differ- 

 ent kinds of cells that compose the organs of the body. In the 

 cells throughout the animal, assimilation occurs and certain types 

 of food may be stored, as is glycogen in liver cells and fat in 

 adipose tissue. 



During its stay out of water, the frog is carrying on respiratory 

 movements of the nostrils, floor of mouth, and glottis. These 

 movements are the results of muscular cell activities, and are pro- 

 duced and controlled as a result of nerve-cell reactions which are 

 dependent, in turn, upon the production of carbon dioxide by cells 

 in all regions of the organism. Oxygen is forced into the lungs by 

 these respiratory movements and passes through the walls of the 

 lungs into the blood, where it enters the red corpuscles and com- 

 bines with haemoglobin. In capillaries throughout the body, 

 oxygen leaves the red cells and the blood stream to enter the 

 protoplasm of all types of cells, where it produces the oxidative 

 reactions of metabolism. As a result of metabolism, excretions 

 are produced and, reaching the blood by diffusion, are eHminated 

 from the body through cells in the liver, kidney, lungs, and skin. 

 A continuation of this discussion would only add further examples 

 of the same kind. Thus we see that the general activities of ani- 



