142 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VII, II
A new and rather disturbing factor was introduced into the scientific study of
human rickets in 1919, with the discovery that the disease could be cured in child-
ren by exposing them to ultra-violet radiation and the realisation which came with
it that many hitherto unexplained observations on human rickets, such as the
seasonal increase in this disease in late winter and early spring after deficient sun-
light, its high incidence in Northern countries with less sun-light and in slum
children in smoky cities, could be best correlated with this discovery of the cura-
tive effect of sun-light. There were thus two mysterious indeterminates in the
picture, namely, the vitamin D of cod-liver oil and ultra-violet radiation, and this
led to various speculations, one of the most attractive and popular theories being
that ultra-violet radiation synthesized this vitamin in the body of the animal.
The right trial was blazed at last by the experiments of Hess, Steenbock and others
in America who made the important observation that food freed from vitamin
D could be endowed with anti-rachitic properties by irradiation. The progress
of the search for the particular constituent of the food which was activatable in
this way, resulting finally in the discovery that it was the natural sterols chole-
sterol in animals and phytosterol in vegetables—associated with the fatty parts
of the food, was one of the fascinating romances of modern bio-chemistry.
With this identification of vitamin D with the irradiation product of a sterol,
vitamin D entered the stage of a chemical problem and the general features of the
molecular structure of sterols characterized by a system of four reduced carbon
rings (cyclopenteno-phenanthrene) with one or more unsaturated linkages and an
alcoholic hydroxylic group. The examination of the various natural sterols and
their activation by sun-light, leading to the discovery of a particular member of
this class found in fungi known as ' ergosterol' which was activated more power-
fully on exposure to light than any other material which had so far been tested,
marked the next important stage in these studies, ergosterol being identified with
' pro-vitamin D '. From this time onwards three groups of workers in England,
Germany and Holland competed for the honour of isolating the vitamin itself, the
purified vitamin obtained in a crystalline condition and having practically the
same physical constants and anti-rachitic activity being announced by them at
about the same time. (The methods by which the different preparations were made
by the English, the Dutch and the German schools, and the separation from the
distilled crystalline material of a pure alcohol which was named " calciferol ",
having the highest biological activity of any thing previously tested by fractional
crystallisation of the 3 : 5 dinitrobenzoic ester, by Bourdillon and his co-workers
in the National Institute of Medical Research at Hampstead, London, were then
described.)
IRRADIATION OF ERGOSTEROL
The next advance was made when it was realised that the irradiation of
ergosterol produced a series of photo-isomerides named Lumisterol, Tachysterol,