The Preoptic Region of the Brain and its Relation to Ovulation 109 



depressing the extralemniscal reticuloactivating system. Critchlow later 

 observed that lesions which selectively destroy the mammillary peduncle 

 tend to block ovulation in rats, supposedly by destroying the fibers of that 

 system which ascend into the posterior hypothalamus. Sawyer, Critchlow 

 and Barraclough proposed that the reticular formation may act permissively 

 upon hypothalamic mechanisms that are more specifically in control of the 

 release of ovulating hormone from the hypophysis. It would be interesting 

 to test the effect of interrupting the mammillary peduncle on the results 

 of preoptic stimulation. Very possibly such lesions would have no more 

 effect than the addition of atropine to the pentobarbital-blocked rat. The 

 present data from experiments with atropine indicate that the introduction 

 of this second "blocking agent" does not elevate the preoptic threshold 

 above that seen with pentobarbital alone. 



Unavoidably the stimulation experiments supply no firm assurance that 

 the preoptic region normally plays any role in ovulation. It is within the 

 realm of possibility that stimulating that part of the brain simply transmits 

 impulses back to the tuberal region by fibers that do not ordinarily function 

 in this way. One is reminded of Sawyer's observation (30) in rabbits sub- 

 jected to the combination of histamine and a low dose of pentobarbital, 

 which set up a characteristic pattern of electrical activity in the rhin- 

 encephalon. This was followed by ovulation. Under these extraordinary 

 circumstances the olfactory bulbs and their connections to the hypothalamus 

 were essential participants, whereas in coitally-induced ovulation the 

 olfactory bulbs can be spared (3). Although the fact is generally recognized 

 that bilateral lesions in the anterior hypothalamus characteristically result 

 in constant estrus in rats (1, 16, 18, 22, 25, 34) as well as in the guinea-pig 

 (8), it is also recognized that ovulatory cycles can be restored in such animals 

 by the administration of progesterone. Studies by Kawakami and Sawyer (24) 

 demonstrate that in rabbits the effects of progesterone are widespread within 

 the brain, influencing thresholds in the mid-brain reticular system, hypo- 

 thalamus and rhinencephalon. Greer (18) reported that many of his lesioned, 

 progesterone-treated rats not only regained cyclic function while they were 

 receiving the steroid, but continued to cycle after treatment was withheld. 

 Thus, it appears that if cells and/or fibers of transit within the preoptic region 

 do take part in the normal process of pro-ovulatory stimulation of the 

 adenohypophysis their roles are not obligatory. 



In the face of this uncertainty, we can nevertheless make use of the preoptic 

 region in several ways. The data at hand bring out several new points of 

 interest. It is now adequately demonstrated that on diestrus day 3 of the 

 5-day cycle the hypophysis is already prepared to release the full quota of 

 ovulating hormone and will do so whenever it receives the necessary signal 

 from the nervous system. The physiological "block" that ordinarily delays 

 ovulation in these rats for another 24 hr is some factor operating within 



