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hormones, as they are often called, are now in the limelight of the physiological stage. Instead of being emptied to the outside through a duct, internal secretions are discharged into the blood

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Fig. 78—Diagram of the spinal cord of a mammal and its connection with the brain:
a, afferent, or sensory, nerve; b, part of brain; e, efferent, or motor, nerve fiber;

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g, spinal ganglion on dorsal root of a spinal nerve;

gr, gray matter of spinal cord; m, m, muscle fibers; nc, nerve cell; s, sense organ. The arrows indicate the courses taken by the nervous impulses. (Modified from W. Mills.)

or lymph. So important are several of these internal secretions that the removal of the organs by which they are produced invariably results in death.

The idea of internal secretions, which was emphasized by

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Claude Bernard in connection with his work on the glycogenic function of the liver, was brought into especial prominence by the experiments of Brown-Sequard. This versatile and somewhat eccentric investigator found that extracts of the testis injected into the blood appeared to exert a stimulating influence on the body. Later investigation has not altogether confirmed Sequard's conclusions, but it has been shown, without any doubt, that the testis forms an internal secretion, which profoundly influences many of the sexual characters by which males differ from females. As a result of practices long prevalent in certain oriental countries, it has been known for many centuries that, if the testes of male human beings are removed early in life, there is a scanty development of the beard, the voice remains high-pitched, and the general form of the body becomes more like that of a woman. Parallel phenomena are observed in the lower animals. Castration of young male deer prevents the development of horns, which normally characterize only the male sex. Steinach, who removed the testes of young male rats and grafted ovaries in their stead, found that these males failed to acquire the large size, rough hair, and pugnacious disposition that usually occur in male rats; on the contrary, the males exhibited a functional development of the mammary glands and the general behavior of the opposite sex. If testes are ingrafted in place of the ovaries in a female rat, she develops the larger size, rough hair, pugnacity, and sex instincts of the male.

These striking phenomena are readily explicable on the assumption that the sex glands produce substances which are given off into the blood and exert a specific influence on the development of certain bodily organs. The sex hormones not only modify physical development, but they profoundly affect instincts, disposition, and the character of the emotions. A feminized male rat will exhibit true, maternal solicitude toward the young of his species, and castrated roosters have been described as sitting upon eggs and brooding young chicks. Psychologically, the differences between males and females appear to be due mainly to the endocrine glands.

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