552 ANATOMY OF THE FCETUS. 



unless excited into growth by some peculiar action, such as the loss or 

 atrophy of the testes. 



Their base is somewhat elliptical, the long diameter corresponding with 

 the direction of the fibres of the pectoralis major muscle ; and the left 

 mamma is generally a little larger than the right. 



Near the centre of the convexity of each mamma is a small prominence 

 of the integument, called the nipple, which is surrounded by an areola 

 having a coloured tint. In the female before impregnation, the colour of 

 the areola is a delicate pink ; after impregnation, it assumes a brownish 

 hue, which deepens in colour as pregnancy advances ; and after the birth 

 of a child, the brownish tint continues through life. 



The areola is furnished with a considerable number of seUparous folli- 

 cles, which secrete a peculiar fatty substance for the protection of the deli- 

 cate integument around the nipple. During suckling these follicles are 

 increased in size, and have the appearance of small pimples, projecting 

 from the skin. At this period they serve by their increased secretion to 

 defend the nipple and areola from the excoriating action of the saliva of 

 the infant. 



In Structure, the mamma is a conglomerate gland, and consists of lobes, 

 which are held together by a dense and firm areolar tissue ; the lobes are 

 composed of lobules, and the lobules of minute csecal vesicles, the ulti- 

 mate terminations of the excretory ducts. 



The excretory ducts (tubuli lactiferi), from ten to fifteen in number, 

 commence by small openings at the apex of the nipple, and pass inwards, 

 parallel with each other, towards the central part of the gland, where they 

 form dilatations (ampullae), and give off numerous branches to ramify 

 through the gland to their ultimate terminations in the minute lobules. 



The ducts and csecal vesicles are lined, throughout, by a mucous mem- 

 brane, which is continuous at the apex of the nipple with the integument. 



In the nipple the excretory ducts are surrounded by a tissue analogous 

 to the dartos of the scrotum, to which the power of erectility of the nipple 

 seems due. There is no appearance of any structure resembling erectile 

 tissue. 



Vessels and Nerves. The mammae are supplied with arteries from the 

 thoracic branches of the axillary, from the intercostals, and from the in- 

 ternal mammary. 



The Lymphatics follow the border of the pectoralis major to the axillary 

 glands. 



The Nerves are derived from the thoracic and intercostals. 



CHAPTER XII. 

 ANATOMY OF THE FCETUS. 



THE medium weight of a child of the full period, at birth, is seven 

 pounds, and its length seventeen inches ; the extremes of weight are four 

 pounds and three quarters, and ten pounds ; and the extremes of measure- 

 ment fifteen arid twenty inches. The head is of large size, and lengthened 



