63. 



39. The whole thorax is covered on the inside with 

 a firm, whifc membrane, called the pleura. It is dou- 

 ble throughout, consisting of two folds : the inner- 

 most whereof has a smooth surface, that it may not 

 hurt the tender substance of the lungs ; the surface of 

 the outer is rough and uneven. From the pleura rises 

 the mediastinum, which is a double membrane, that 

 divides the lungs and the cavity of the thorax length. 

 ways into two parts. 



40. On the slightest observation we cannot but ac- 

 knowledge the consummate wisdom, wherewith the 

 external parts of the middle cavity are formed, for 

 beauty, as well as for the defence of the internal. 

 This is commodiously connected with the head, by the 

 neck. The breast, or fore part of (he thorax, which 

 begins at the throat, and en<is at the sternon, or bi east- 

 bone, is an admirable guard to (he noblest parts. To 

 the same end serve the shoulder-blades and the back- 

 bone, as well as to support the whole fabric. 



The breasts consist of numberless oval glands, in. 

 ter mixed \\ith globular vessels of fat. Their ducts, 

 as they approach the nipple, unite together, till ' they 

 form eight or more sriial! pipes, communicating with 

 each other by cross canals, which are of great use, 

 when some of them happen to be obstructed. These 

 tubes are in some parts narrower, in some wider, so as 

 to form cells, which hinder the efflux of the milk. The 

 paps consist chiefly of the concurrence of these tubes, 

 but with a glandi.lous substance intermixed. There 

 are likewise joined herewith abundance of fibres, from 

 the external teguments of the breasts, by means of 

 which the tubes are constringed, and the motion of 

 the milk is modified. 



In virgins, the glands of the breasts are so contract- 

 ed, that no blood can enter them. Hut when the 

 womb swells with the foetus, and compresses the de- 

 scending trunk of the great artery, the blood forces its 

 way into them. They admit thicker and thicker 

 serum, till after the birth ? they run with a thick 

 milk. 



