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IN Part IV of this series [Blackwood and Stirling, 1932, 1] some consideration was given to certain aspects of the phosphorus metabolism of the mammary gland. While there were indications in the data obtained that the experimental proof of the phosphatide theory of fat secretion given by Meigs et al. [1919] could not be substantiated, it was felt that in the absence of lipin-phosphorus determinations such a conclusion was not then entirely justified. The present series of experiments was planned to explore more fully the postulated relationship between phosphorus in the blood and the fat metabolism of the mammary gland during lactation. At the same time it was thought desirable to determine the level of the total alcohol-ether-soluble fraction of the whole blood and plasma together with the cholesterol content during the passage of the blood through the active and inactive mammary gland of the cow. EXPERIMENTAL.The technique of blood sampling was that employed in the earlier work of this series but it may be desirable to give our experience in detail. Samples of blood were drawn simultaneously from the radial artery and from the jugular and abdominal subcutaneous (mammary) veins of lactating and non-lactating cows. Arterial puncture in non-aesthetised cows is very frequently attended by disturbance. Where this seemed probable the animal in question was abandoned and a more suitable subject chosen. Finally it was found that by preliminary gauging of the size of the radial artery, it was possible to select animals for experiment such that disturbance was reduced to a minimum. The sample from the jugular vein was obtained with slight constriction either by finger pressure or by momentary application of a "choke" rope. In every case the entire process of jugular sampling was complete in about 30 seconds. No special measures were required for the mammary blood sample. No difficulty was ever experienced with either of the venous samples and no oedema resulted with this technique. Only those cases where the entire procedure of arterial, jugular and mammary sampling was completed within 4 minutes are included in the work on milk secretion. Each sample of 60 ml. volume was drawn into a dry sterile glass bottle containing 0-1 g. of sodium oxalate as anticoagulant. The plasma was separated immediately and the various fractions examined.

THE object of the present series of papers is to effect a correlation of our knowledge of milk secretion and of the chemistry of the blood.Since the process of milk secretion involves complex molecular transformation within the secreting cells of the mammary gland, it is a matter of physiological importance to determine which constituents of the blood are selected by the gland for conversion into milk protein, carbohydrate and fat. Such data are also of immediate practical importance since effective control of the milk production of animals requires a co-ordination of diet and milk yield and, therefore, an adequate knowledge of the relationship between active gland and blood-stream-the intermediary between gland and digestive tract.The secretory process of the mammary gland is extremely sensitive and can best be studied in the intact animal. The ideal method, therefore, of determining whether any given blood constituent takes part in the secretory process is to compare the concentration of that constituent in the blood before entering and after leaving the gland. A diminished concentration in the efferent blood would indicate utilisation by the mammary gland.The mammary glands of the cow are drained by the abdominal subcutaneous veins which are superficial and therefore readily accessible without disturbance to the animal. The arterial blood, however, is supplied by the mammary artery which is deeply embedded in the tissues and is inaccessible without serious disturbance to the animal.The problem of securing samples of arterial blood with which the comparison of analytical figures for mammary venous blood is justified is therefore one of considerable practical difficulty and was first encountered by Kaufmann and Magne [1906] who wished to investigate the consumption of blood-glucose in the gland under varying conditions of activity.These workers chose the concentration of sugar in the blood of the jugular vein as a standard with which to compare the concentration of sugar in the mammary venous blood. In this assumption that jugular blood may be taken

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With 1 Figure) PUBLISHED evidence regarding the effect of pasteurization on the assimilation and retention of nitrogen, phosphorus and calcium from milk is both fragmentary and conflicting. The subject has been carefully reviewed by Stirling & Blackwood(i). It is remarkable to find that, although a number of investigations have been made in which dogs, pigs and human infants have been used as experimental subjects, calves have not been employed for this purpose. This is the more striking since it might be anticipated that any alterations induced in cow's milk by heating would be most readily demonstrated on the young of the same species.The investigation described in this paper was intended to make good this omission. Three series of experiments are reported. In the first series several calves were fed on fresh and dried skim milk as well as on raw and pasteurized whole milk. The results obtained with these calves will be described elsewhere. In the second and third series only raw and pasteurized whole milk were fed. TECHNIQUEThe calves-all males-were obtained from tuberculin-tested Ayrshire herds. They received their dam's milk for at least 3 days and were then given raw milk up to the tenth day, the quantities being increased daily until they were receiving an amount equal to 10 per cent of their body weight. During this preliminary period they were housed in metabolism cages. The metabolism cages were constructed of wood and were provided with false bottoms which permitted the separate collection of urine and faeces. It was found in the first series of experiments that the calves tended to gnaw the exposed woodwork of the cages. This was therefore covered with sheet metal.At 10 days old the calves were placed on experimental diet. This consisted of either raw or pasteurized whole milk. Both types of milk were derived from a Grade A (T.T.) herd. The pasteurized milk was heated in a tinned iron container which was immersed in a large water-bath. The time taken for the