66 The Biochemistry of Semen 



changes during freezing. Glycerol has long been recognized in 

 protein and enzyme chemistry as a convenient, 'stabilizing' agent 

 which combines the properties of a protein solvent with the ability 

 to protect the protein from denaturation caused by temperature 

 changes. It has been shown to prevent the heat coagulation of serum 

 and egg albumin (Beilinsson, 1929), and is in use in the cold storage 

 of egg yolk owing to its solubilizing action on lipoproteins (cf. 

 McFarlane and Hall, 1943). Lavin, Northrop and Taylor (1935) 

 used glycerol in their study of pepsin at -100°; Keilin and Hartree 

 (1949) discovered that the presence of glycerol at very low tempera- 

 tures intensifies the absorption spectra of haemoproteins some 

 twenty-five times. This in turn, made it possible to demonstrate for 

 the first time, the spectrum of cytochrome in human spermatozoa 

 (Mann, 1951^). The protecting influence of glycerol on sperm col- 

 loids may well be linked with the electrolyte- and water-binding 

 properties of this substance. In this connection, an observation by 

 Luyet deserves to be mentioned, on the existence of a definite rela- 

 tionship between the water-binding capacity of different solutes 

 used for vitrification, and the temperature at which devitrification 

 takes place on thawing; the devitrification temperature of glycerol 

 is in the neighbourhood of -70° (Miner and Dalton, 1953) which is 

 below that of sucrose, fructose, gelatin, various gums and most 

 other solutes. 



Role of hormones 



Among factors which influence the production of semen in man 

 and animals, hormones rank paramount in importance. The forma- 

 tion, output, and composition of ejaculated semen are the outcome 

 of a concerted action of several endocrine organs, with the pituitary 

 gland and the testicular interstitial tissue in dominant positions. 



Apart from the direct gonadotrophic activity due to the game- 

 togenic and interstitial-cell stimulating hormones, the anterior lobe 

 of the pituitary gland exerts an indirect influence upon the male 

 reproductive organs, through interaction with the thyroid gland and 

 the adrenal cortex. The anterior hypophysis itself depends on stimuli 

 from the central nervous system and responds in a particularly sen- 

 sitive manner to impulses transmitted through the optic nerves. 

 The seasonal fluctuations in the intensity of light impulses, relayed 



