GENERAL ZOOLOGY 



tenance of pregnancy, that is, the function of the placenta, depends on the 

 progesterone released by the corpus luteum. In others, including man, the 

 chorionic portion of the placenta becomes an endocrine organ secreting a 

 gonadotropic hormone in addition to progesterone. This origin of a new 

 site for the production of progesterone makes possible an extended period of 

 gestation resulting in the birth of more fully developed young. Birth is 

 correlated with a reduction in the amount of progesterone released to the 

 blood. 



Some estrogen is secreted during pregnancy, and it stimulates growth in the 

 mammary glands. After growth has occurred, the glands differentiate their 

 secretion under the influence of progesterone. At the time of parturition, 

 the mammary glands become responsive to the luteotropic hormone and milk 

 becomes available for the suckling newborn. Changes in the behavior pattern 

 are also conditioned by the luteotropic hormone which releases, so to speak, 

 the nesting and maternal instincts of the expectant mother. 



This sequence of events so beautifully coordinated by hormonal and central 

 nervous mechanisms insures the continuity of the species. Insofar as the 

 reactions of the reproductive tract are concerned, the effect of the ovarian 

 hormones is on fundamental metabolic processes. The first responses of the 

 uterus to estrogen are an increase in vascular bed and enhanced permeability 

 of capillaries together with a rise in oxygen consumption. Then there occurs 

 a marked shift of water and ions to the uterine wall. Utilization of glucose 

 increases and, next, synthesis of new protoplasm followed by increased cell 

 division. Growth of the uterus has occurred. How do these particular steroid 

 molecules set in train these adaptive processes? Since the molecules are 

 believed to be too large to enter the cell, current hypotheses assume that the 

 initial response occurs at the cell membrane. The final elucidation awaits 

 further research, perhaps new methods. 



Regulation of Breathing and Body Temperature. We have seen 

 in the special events of reproduction how several interrelated factors regulate 

 the sequences in an adaptive fashion. This is equally impressive in continuous 

 regulations of vital functions such as breathing and, in warm-blooded animals, 

 maintenance of a constant body temperature. 



The regulatory mechanisms for the control of breathing are conditioned by 

 the amount of carbon dioxide in the blood. In higher vertebrates, ventilation 

 of the lungs occurs as a result of contraction of thoracic muscles aided, in 

 mammals, by the diaphragm. The rhvthmicity of the respiratory movements 

 is a reflection of the rhythmic activity of the cells of the respiratory centers 

 located in the medulla of the brain. These cells are very sensitive to minute 

 changes in the amount of carbon dioxide in the blood flowing through the 

 capillaries about them. A slight elevation in the carbon dioxide tension 

 stimulates the cells of the respiratory centers, and impulses are established 

 which are finally conducted along the thoracic spinal nerves to the muscles 

 between the ribs; contraction of these muscles pulls air into the lungs. As 

 the lungs fill, tension builds up in their walls and nerve impulses are estab- 



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