lo Feb., 1909.] Elements of Afiimal Physiology. 81 



THE ELEMENTS OF ANIMAL PHYSIOLOGY. 



W. A. Osborne, M.B., D.Sc, Professor of Physiology and Histology, 

 Dean of the Faculty of Agriculture in the University of Melbourne. 



{Continued from page 32.) 



XVII. — Reproduction— cOTiftjmec?. 



LACTATION. 



The mammary glands vary in number in different animals. In_ the 

 cow there are really two, for the udder, though apparently single, is divided 

 completely by a fore and aft partition of strong connective tissue which 

 completely separates the two glands. Each gland has one or more main 

 ducts and teats. The mammary glands increase in size at puberty but 

 such increase is largely due to connective tissue and fat. When, however, 

 the animal becomes pregnant the true glandular matter undergoes great 

 increase. This response of the mammary gland to pregnancy has been 

 proved to be due to hormone stimulation, but a doubt has arisen as to the 

 origin of the hormones. Probably such origin is twofold — from the ovary 

 and from the foetus. In an animal in the wild state no secretion of milk 

 takes place, except when suckling, but, in domesticated animals, milk 

 secretion may occur throughout pregnancy and even in the virgin state. 

 Certain evidence points to the fact that from the foetus or its placenta a 

 hormone enters the blood stream which, reaching the gland, holds secretion 

 in check. The moment the placenta is detached this restraining agent is 

 removed and the gland begins to secrete actively. In the highly selected 

 and domesticated milch cow pregnancy does not stop the formation of milk 

 though it certainly lessens the amount secreted in the later months. 



The mammary gland is a true gland and its product a true secretion. 

 The chief ingredients of milk do not appear preformed in the blood but 

 are manufactured by the gland cells out of the nutriment supplied by the 

 blood. In one respect the secretory epithelium of the mammary gland is 

 unique. The lining cells, when active, not only produce a secretion but 

 apparently break up, in whole or part, and pass into the secreted fluid. 

 Milk is to a certain extent a cell pulp as well as a fluid secretion. The 

 cells that are thus mutilated or lost are speedily repaired or replaced by 

 cell growth and subdivision. The mammary gland has a fine substructure 

 of connective tissue, embedded in the meshes of which lie the true gland 

 cells. It is plentifully supplied with blood vessels, lymphatics, and 

 nerves. The ducts leading from the small gland follicles or acirii join 

 together to form larger ducts, and so on until one or two large ducts are 

 formed which pass each through a teat. One peculiarity of this duct 

 system is the presence of reser\'oirs, the largest being just at the root of 

 the teat. Another curious and important fact is, that in the branching part 

 of the duct system, sphincter muscles occur over which the animal has some 

 control. The teat is also well supplied with sphincter muscles. Closure 

 of the ducts can occur reflexly through fright, unfamiliar surroundings, or 

 oestrus, but it is certain that cows can acquire a pernicious habit of volun- 

 tarily holding their milk on the slightest provocation. In some cows the 

 sphincter of the teat is so toneless that the mere pressure of the milk in 

 the udder is sufificient to force it open. If milk is allowed to stagnate in 

 the ducts and their reservoirs there is a partial back absorption of some 



