STUDIES ON THE MAMMARY GLAND 359 



A very good time to make these observations on the albino 

 rat is about the tenth to the fourteenth day of Ufe. As pointed 

 out by Jackson ('12), the mammary glands (nipples) are very 

 conspicuous at this time. After the first two weeks have passed, 

 it is very difficult to make accurate observations as the glands 

 are well covered with dense hair. During pregnancy and the 

 period of lactation the glands again become very conspicuous. 



Just medial to the apex of each nipple is a single opening which 

 leads into one large duct. This duct after reaching the tela sub- 

 cutanea turns almost at right angles and courses through this 

 layer parallel to the surface. Instead of receiving a large number 

 of tributaries from all directions, this single duct at first receives 

 only a small number of tributaries usually from a single direc- 

 tion. Figure 2, drawn from a cleared preparation of the integu- 

 ment of a rat two weeks old, will serve to show the general direc- 

 tion taken by the ducts of each gland. Here it may be observed 

 that the main duct of the first thoracic gland extends cephalad, 

 then breaks up into numerous branches. No ducts are seen to 

 pass out in any other direction from the nipple. The general 

 direction of the ducts of the second thoracic gland is somewhat 

 different as they pass almost directly laterad from the nipple. 

 In a number of cases, however, these ducts were found to extend 

 somewhat latero-cephalad. A larger number of branches lead 

 from the third duct than from either of the other thoracic nipples. 

 The ducts of the third take the same direction as do those of the 

 second thoracic gland. The abdominal gland, whose duct shows 

 a greater amount of branching than any of the others, sends its 

 branches in a caudo-lateral direction. The main duct of the 

 first inguinal gland, after passing a short distance caudo-laterad. 

 breaks up into ramification some branches of which take a 

 cephalic while others take a caudal direction. The duct of the 

 last or second inguinal gland usually sends all of its branches di- 

 rectly caudad. In some specimens, however, a few branches 



Fig. 2 Drawn from a cleared preparation of a two weeks' albino rat (internal 

 view) to show the general arrangement of the nipples and the branching of the 

 mammary ducts. X 4. T.n.l, T.n.2, T.n.3, 1st, 2d and .3d thoracic nipples; 

 A.n.l, abdominal nipple; I.n.U I.n.2, 1st and 2d inguinal nipples. 



