An udder that showed an extreme case of pitting edema after calving was also studied. In this condition, the pressure with the tip of a finger leaves a dent in the surface that persists for several minutes after the pressure is removed. We found that the intense swelling did not appear to invade or to affect the secretory tissue of the udder to any appreciable extent, but was confined chiefly to the space between the glandular tissue and the skin. Opinions differ as to whether the occurrence of the condition is desirable in a dairy cow.

The unsoundness of a practice sometimes followed of making allowances for lost quarters of an udder and adjusting the production upward in proportion to the number of quarters lost, when comparing production records of individual cows, was emphasized in a study of an udder that failed entirely to yield milk from the two rear quarters.

The cow was a Jersey that calved at 2.5 years. She produced, entirely in the front quarters, 7,223 pounds of milk and 410 pounds of butterfat in 365 days during her first lactation period. Research by us and others shows that the front quarters of an udder produce on an average about 40 percent of the total milk yield. On that basis, one might figure that, since only 40 percent of her udder was functioning, this cow's producing ability as a 2-year-old was 18,057 pounds of milk and 1,025 pounds of butterfat. Further correction for age would have raised her estimated mature equivalent capacity to 23,294 pounds of milk and 1,282 pounds of butterfat. A cow like that would be worth a lot of money, but we had no reason to believe she possessed an inheritance for production that even approached that high level. When she was slaughtered, it was found that the secretory tissues of the front quarters had expanded backward and occupied most of the area above the rear teats normally occupied by rear-quarter tissue. She gave us evidence that nature sometimes compensates in large measure for the loss of one or more quarters, particularly when the quarters are lost early in life.

Occasionally, infections occur in one or more quarters of a young calf's udder. In one instance, the infection was first noted at 18 days of age. In another case the calf was a month old. Others have been 3 to 5 months old when the infection was first noted. In every case on record where pus discharge from the teat occurred in the young calf, the affected quarter has failed to yield milk from the beginning of lactation. Usually these quarters filled out like the others before calving, but after calving no milk could be obtained, and soon the affected quarter became shrunken to a fraction of the size of the opposite quarter. Post mortem examination of some of these udders showed that the secretory tissue of the affected quarter developed and probably secreted milk but scar tissue in the region of the cistern prevented the passage of any milk. As a result, involution quickly followed and the quarter became permanently "blind."

In hundreds of udders sectioned for post mortem anatomical studies, none has been found to contain a cancerous growth. This is in agreement with other findings that cancer is virtually nonexistent in the cow's udder. While it is difficult to understand why mammary cancer, a disease of such high incidence in humans and other species, should be so nearly nonexistent in the cow, it is gratifying to know that this is true of the gland in which one of our most important foods is produced.

The yield and rate of milking of each of the four quarters of the udder, in cows milked by machine, has been determined for 94 cows. The front quarters yielded 41.8 percent and the rear quarters yielded 58.2 percent of the total. Rates of milking varied during the course of a milking, and from one milking to another if there were differences in total milk yields. Higher milk yields resulted in higher rates in pounds a minute. However, higher milk yields—whether from separate quarters or for the entire udder—required a little longer milking time, but the percentage increases in time were much less than the percentage increases in milk yield. These results have an important bearing on fast-milking procedures now being advocated in many places as desirable from the standpoint of economy of time and good dairy-herd management.

The importance of an abundant blood circulation through the udder of lactating cows has long been stressed. A thorough study of the course of arterial and venous circulation has been made. The results indicate that the internal veins are adequate in size to take care of the return of blood from the udder. In this connection, it was found by studying 106 Holstein and 89 Jersey cows that neither an abundance of veining on the abdominal wall or on the surface of the udder nor the presence of large "milk wells" indicated high milk-producing ability.

Fundamental information has been obtained with regard to the structures by which the udder is suspended from the body. This information has made it possible better to understand why the udders of high-producing cows—especially udders that are fleshy, or edematous—may overtax the structures by which they are supported, and become broken down and misshapen. If inherited weaknesses in the suspensory apparatus and excessive weight of the udder and its contents are the chief factors contributing to the broken-down condition, the selection of breeding stock with an inheritance for well-supported udders, and frequent milking of heavy producers, should be the most effective preventives.

THE AUTHORS

W. W. Swett is a dairy husbandman in the Division of Dairy Cattle Breeding, Feeding, and Management of the Bureau of Dairy Industry. He has been in charge of the Bureau's research pertaining to the interrelationships between body form, anatomy, and udder structure in dairy cattle since its beginning in 1922.

C. A. Matthews, also a dairy husbandman in the same Division, has assisted in most of the studies and has been responsible for carrying out a number of them.