18 THE MICROSCOPE 



divisions of the bronchial tubes, which convey the air into the 

 lungs, end in elongated dilatations, about one-fortieth of an inch 

 in diameter, each of which is made up of numerous little cells or 

 sacs opening into the cavity of the dilatation — which are called 

 air-cells — and the very thin walls of these air-cells carry the wide, 

 thin-walled, and closely-set capillaries, into which the ultimate 

 divisions of the pulmonary arteries pour their blood, so that the 

 blood is exposed on both sides to the air, being separated only by 

 the delicate pellicle which forms the wall of the capillary and the 

 lining of the air-sac ; and while this exposure is going on, the 

 blood parts with its carbonic acid and absorbs fresh oxygen for 

 the proper support of the vital functions. 



Again, in the liver^ we find that the portal vein, which conveys 

 the blood from the abdominal viscera, breaks up into minute 

 branches, which map out the whole liver into small oval divisions 

 or lobules, and from these branches proceed on every side minute 

 capillaries, filling up nearly the whole substance of the liver, and 

 finally joining a vein, which with outspread branches occupies the 

 centre of each lobule, and carries off the blood from the liver, 

 after the secretion of bile has been effected by means of the 

 hepatic or bile-cells, which fill up all the interspaces between the 

 before-named capillaries, and which discharge their contents into 

 the minute bile-ducts, which lie in the spaces between the lobules ; 

 while in the kidneys the vessels which supply these organs, form 

 little vascular tufts or balls — called malpighian corpuscles — each 

 of which lies in a little flask-shaped sac or capsule, proceeding 

 from the walls of certain small tubules, which lie in the substance 

 of the kidney, and which are lined with nucleated gland-cells, by 

 means of which the separation of the urine from the blood is 

 effected. 



The microscope has also revealed to us the wonderful provi- 

 sion for regulating the supply of blood to each part. Every 

 artery — even the smallest — is seen to be encircled by numerous 

 muscular fibres, which have the power of contracting the size of 

 the vessel, and thus diminishing the quantity of blood passing 

 through it. And these muscular coats are seen to be supplied 

 with numerous fine nerve-fibres, by which the degree of contrac- 

 tion is determined. 



