THE FROG — A REPRESENTATIVE VERTEBRATE 419 



and the frog retracts its leg. The response to this kind of a stimulus is 

 always the same, it happens very rapidly, and need not involve much, 

 or any, passage of impulses up and down the central nervous system. A 

 great deal of the integration of the body is achieved by reflexes occurring 

 subconsciously either in the cord or in the brain. 



Some regions of the brain have evolved as integration centers for 

 impulses coming in from major sense organs. The telencephalon and 

 diencephalon of frogs are concerned primarily with the integration of 

 olfactory impulses. When these regions are destroyed, the frog does not 

 move spontaneously, presumably because it cannot respond to olfactory 

 or visual stimuli. (Generally optic tracts also are destroyed in this 

 operation.) However, the frog does maintain its posture, and can feed, 

 jump and swim upon proper stimulation. The optic lobes integrate 

 impulses of sight, but some other sensory impulses are projected to the 

 optic lobes in frogs and other lower vertebrates, so this region has, to 

 a limited extent, the over-all integrative function assumed by the 

 cerebral hemispheres in higher vertebrates. Electrical stimulation of the 

 area can, for example, induce movement of the limbs. Its destruction 

 prevents response to optic impulses, and also removes a dampening or 

 inhibiting effect upon spinal reflexes. The cerebellum and medulla 

 receive impulses from the ear, and also sensory impulses from most 

 muscles which indicate their present state of activity. In addition, 

 respiratory movements and some other vital activities are controlled 

 reflexly in the medulla. When these regions are destroyed the frog loses 

 its ability to maintain its posture, or right itself when turned over. 

 Muscular coordination is impaired, though not as much as in birds and 

 other vertebrates with a larger cerebellum. Feeding is impossible and 

 respiratory movements stop. Spinal reflexes continue for a while, but 

 the animal eventually dies. 



192. Endocrine Glands 



Some of the integration of metabolic processes and other vital 

 activities is controlled by the secretions (hormones) of the endocrine 

 glands. Although chemical integration in the vertebrates is discussed 

 more fully later (Chapter 30), two major endocrine glands of the frog 

 may be brieffy considered. 



The pituitary gland, which is attached to the floor of the brain, is 

 often regarded as the master endocrine gland, for it produces a variety 

 of hormones including some that regulate ^the activity of many other 

 endocrine glands. Among its hormones are intermedin, which helps 

 control skin coloration; a gonad-stimulating hormone, which stim- 

 ulates amplexus and the release of the gametes; and a growth-stimulat- 

 ing hormone, which controls growth of the larvae. 



The thyroid gland is paired in frogs, and located on each side of 

 the posterior part of the hyoid apparatus. Its hormone, thyroxin, is 

 necessary for metamorphosis from larva to adult, and for an adequate 

 level of metabolism in the adult. 



