276 



HYPOPHYSIS AND GONADOTROPHIC HORMONES 



the eminentia. It was his thought that the 

 eminentia has a sensory innervation which 

 registers the content of adenohypophyseal 

 hormones emanating from the pars distalis. 

 The view seems to be completely untenable 

 on anatomic grounds alone. 



The capillaries in the primary plexus of 

 the portal vessels show an unusual arrange- 

 ment in most mammals. From the net which 

 spreads thickly over the surface of the me- 

 dian eminence and the neural stalk, capil- 

 lary loops and tufts protrude into the wall 

 of the median eminence (Green, 1948; 

 Xuereb, Prichard and Daniel, 1954a). They 

 are not in close proximity with the large 

 nerve tracts of the hypothalamo-hypophys- 

 eal neurosecretory system. In birds and 

 mammals they are approached but not 

 closely surrounded by sparse fibrous ele- 

 ments, the nature and origin of which are 

 obscure. 



There is a mounting body of circum- 

 stantial evidence suggesting that the hypo- 

 thalamus exerts a large measure of control 

 over the secretory functions of the anterior 

 pituitary. Such control might be effected 

 either through direct nervous connections, 

 the evidence for which (as we have just 

 seen) is scant, or through a relay chain 

 made up of a neural and a vascular link. 

 Anatomically, at least, the latter exists in 

 the form of the hypophyseal portal system. 



The neurohumoral concept as outlined by 

 Harris (1947, 1948a) and Green and Harris 

 (1947) holds that pertinent exteroceptive 

 stimuli impinge upon the hypothalamus as 

 the first way-station in the neurovascular 

 reflex arc. Here the sex-related impulses 

 are integrated with existing blood levels 

 of circulating hormones, thence effector im- 

 pulses travel by nerve conduction to the 

 median eminence and effect the liberation 

 of neurohumors (chemotransmitters). The 

 latter are held to enter the primary plexus 

 and to be distributed to and activate the 

 cells of the pars distalis. A weakness in this 

 theory is at the point of transfer of chemo- 

 transmitters from nerve to vessel in the 

 median eminence. The anatomic relation- 

 ship between the loops or endings of nerves 

 of obscure origin and the capillaries, which 

 lie either on the surface of the median 

 eminence or extend tuft-like into its sub- 

 stance, has not Ix'cn clearlv defined. 



D. THE HYPOTHALAMIC AND HYPOPHYSEAL 

 MEDIATED SEXUAL FUNCTIONS 



1. Environmental Stimuli 



A great number of instances are known in 

 which reproductive processes are condi- 

 tioned by or are dependent on environ- 

 mental factors. Although animals breed at 

 a season of the year that is propitious for 

 the survival of their young — warmth and 

 the availability of food being the major 

 factors — this is not through choice on their 

 part. In many birds and mammals the 

 readying of the reproductive system for 

 seasonal breeding and rearing of young is 

 anchored to alterations in the physical en- 

 vironment, especially to changes in length 

 of daylight — a subject which will not again 

 bear expatiation (for a recent review see 

 Amoroso and Matthews, 1955). Darkness, 

 too, may not be an entirely passive stimulus 

 (Burger, Bissonnette and Doolittle, 1942; 

 Jenner and Engels, 1952; Kirkpatrick and 

 Leopold, 1952) , but this concept has not 

 gone unchallenged (Hammond, Jr., 1953). 

 Experiments involving total darkness, more- 

 over, are often subject to the complication 

 of reduced food intake. The ground squirrel 

 (Wells and Zalesky, 1940) and the white- 

 crowned sparrow (Farner, Mewaldt and 

 Irving, 1953) respond only to changes in the 

 ambient temperature. Propagation in some 

 tropical birds and amphibia is in like man- 

 ner dependent on the beginning of a rainy 

 season. Food becoming more plentiful at 

 this time seems to be the mediating factor 

 for many tropical birds. In England, certain 

 frogs (Savage, 1935) are lured to the ponds 

 by cheraotactic stimuli arising from ripen- 

 ing algae, but spawning is dependent on 

 another sequence of changes induced in the 

 ionic composition of the water by the algal 

 cycle and perceived through the frog's skin. 

 Animals transported to the opposite hemi- 

 sphere, generally adapt their breeding ac- 

 tivities rather quickly to the same charac- 

 teristic season irrespective of when it occurs 

 in the calendar year (for review see F. H. A. 

 Maishall, 1942; Burrows, 1949). In many 

 birds, even with gonads in readiness for 

 the mating season, the actual nesting often 

 requires another complex set of extra-organ- 

 ismal stimuli, such as presence of food, pres- 

 ence of mate, density of colony, suitability 



