602 THE PLEUEA. 



selves, thus forming valves to prevent the escape of the 

 milk when not needed. These ducts, however, have no true 

 valves, and injection shows them to have no communication 

 with each other. 



The interior of the mammary gland, when a section is 

 made, presents a white, fibrous appearance, in which the 

 granular arrangement is not so distinct, unless the ex- 

 amination, it is remarked, be made during lactation. 

 The arteries supplying this gland come from the thora- 

 FIG. 194. cic, the intercostals, and inter- 



nal mammary. The veins are 

 superficial and deep the lat- 

 ter accompanying the arteries. 

 The nerves are derived from 

 the brachial plexus and inter- 

 costals. 



The lymphatics of the mammae 

 are numerous, some of which are 

 traced to the glands of the ax- 

 illa, others pass through the intercostal spaces into the an- 

 terior mediastinum, to the lymphatic glands in this situa- 

 tion ; while others accompany the intercostal vessels to the 

 posterior mediastinum, or enter directly the thoracic duct. 

 Function. To secrete the milk designed for the nourish- 

 ment of the infant. The closest sympathy exists between 

 the mammae and the uterus. 



THE PLEUEA, (Fig. 195.) 



The pleurae form the interior lining and complete the 

 walls of the chest. They are two in number and consist of 

 complete sacs without any opening, and are reflected from 

 the walls upon the viscera which they enclose. The organs, 

 however, are all on the outside and none within the pleu- 

 ral cavity. Each pleura is one continuous membrane, and 

 can be traced throughout its whole extent. That portion 

 covering the walls is called pleura parietalis or costalis, 

 and that covering the lungs, pleura pulmonalis. The 



FIG. 194 represents the Lactiferous Ducts. 



