MAMMARY GLANDS AND LACTATION. 761 



of which the bile is formed, when retained, or the bile itself when 

 taken up, appear, therefore, to be noxious, or even poisonous. 



The combined assimilative, secreting, excreting, and purifying ac- 

 tions of the liver, are consistent with its large size, its general presence 

 in nearly all animals, its marked vascularity, the peculiar source of 

 its blood, the high temperature of its tissue and of the hepatic blood 

 returning from it, and the singular variety of the metamorphic changes 

 which take place in it. So long as its office was supposed to be merely 

 to secrete 2 oz. of solid biliary matter daily, whilst the lungs excrete 

 8 oz. of carbon in the same time, the size and other characters of this 

 gland were not fully explained, especially in its embryo state; but its 

 glycogenic function, and its influence in the process of sanguification, 

 sufficiently account for its pre-eminence amongst all the glandular 

 organs of the body. 



The Mammary Glands and Lactation. 



The human infant, and the young of all Mammalia, are supplied 

 with suitable nutriment for the first months of their existence, in the 

 well-known fluid named milk, secreted by the mammary glands. It is 

 in the female only that these glands yield milk, the process being 

 termed lactation. In the males of mammiferous animals, these glands 

 exist, but their parts are very small. 



The mammary gland, in woman, is a large organ, composed of nu- 

 merous lobes, arranged, in a more or less radiating manner, around 

 the projecting part, named the mammilla, or nipple. The lobes, 

 which may be moved slightly upon each other, are separated by fibrous 

 septa, and are held together by a general investment, stronger on the 

 under side of the gland, where it rests upon the pectoral muscle. 

 Each lobe consists of a number of lobules, possessing the structure of 

 a compound racemose gland, closely resembling that of the parotid 

 gland (Fig. 42, c). The terminal ducts end in clusters of short folli- 

 cles, or vesicles, about 3^0^ f an i nc ^ ^ n diameter, which, when filled, 

 are just visible to the naked eye, and the walls of which are lined with 

 a layer of soft glandular epithelial cells. From these follicles, the 

 smallest lactiferous ducts unite into one or more larger ducts for each 

 lobule, and these join into still larger tubes, called galactopherous 

 ducts, one or more for each lobe. These large ducts, about fifteen 

 in number, run to the centre of the gland, and generally dilate, so as 

 to form temporary receptacles for the milk. The walls of these ducts 

 are composed of a fibrous coat, containing unstriped muscular fibres, 

 and lined by a mucous membrane continuous with the skin. They 

 open at the summit of the nipple, by separate small round orifices, 

 seen at the bottom of little depressions in the skin. The arteries of 

 the mammary gland are numerous, and proceed from many sources; 

 they present a good example of the enlargement of bloodvessels sup- 

 plying a part in which increased activity of function occurs. Numer- 

 ous capillaries surround the terminal vesicles of the gland. The veins 

 and lymphatics are also numerous. So likewise are the nerves^ partly 

 spinal and partly sympathetic, the latter reaching the gland along 

 the arteries. 



