THE THORAX— THE BLOOD. 273 



where the air-tubes and blood-vessels pass into their substance. 

 This portion of the pleura is continuous with the' serous membrane 

 lining the ribs (G Gr, pleura costalis), which thus allows them to 

 expand and contract freely, by allowing one surface to glide against 

 the other. Thus, the pleura on each side covering the lungs, and 

 reflected thence to the inside of the ribs, and the thoracic side of 

 the diaphragm, forms a shut sac or bag, which in the natural state 

 contains only sufficient serum to lubricate its walls ; but in disease 

 this is often increased to an enormous extent, ending in dropsy of 

 the chest, or in a collection of pus when the membrane is greatly 

 inflamed. 



As the walls of the thorax expand by the action of the muscles 

 which move the ribs, as well as by the contraction of the diaphragm, 

 rendering its thoracic surface less convex, the cavity is enlarged 

 and air is drawn in through the trachea, constituting the act of 

 inspiration. On the other hand the contraction of the walls, and 

 the forcing upwards against the diaphragm of the stomach and 

 liver, by the action of the abdominal muscles, reduces the size of 

 the thorax, forces out the air, and induces expiration. The repe- 

 tition of these two actions is known by the general term respiration. 



Before proceeding to describe the heart and lungs, it will be 

 necessary to examine the blood, for transmitting which fluid to all 

 parts of the body the heart and its vessels are formed; while, for 

 its proper aeration, the lungs, windpipe, and larynx, are intended 

 by nature. 



THE BLOOD. 



The blood, supplied from the food by the digestive process 

 hereafter to be described, furnishes all the tissues of the body with 

 a constantly renewed stream of the materials which they severally 

 require, whether for their nutrition or for the functions of secre- 

 tion and excretion performed by the various organs devoted to 

 these purposes. It is necessary, therefore, that this fluid should 

 lie ci mposed of elementary matters capable of combining to form 

 the materials required, or of those substances ready prepared. 

 Thus, the muscles demand for their proper action fibrine and oxy- 

 gen, both of which are largely combined in arterial blood, while 

 the nervous system caunot respond to the calls of its grand centre 

 without having a due supply of fatty matter, also, in combination 

 with the oxygen obtained by respiration, which, however, is not 

 only intended to afford this gas, but also to remove the carbon 

 that would otherwise accumulate to a prejudicial extent. For 

 these several purposes the blood must be supplied with liquid ele- 

 ments by absorption from the digestive organs, and with its oxy- 

 gen, by imbibition through the delicate membrane lining the lungs 

 on which it is spread as it passes through the system of blood- 



