Organs of Respiration and Circulation. 



595 



situation principally by the large vessels which are going to and 

 from them, and also by the windpipe : they are covered externally 

 with a serous membrane, the pleura, which is reflected upon the 

 sides of the thoracic cavity, and forms also the mediastinum before 

 spoken of. The lungs are everywhere free in the chest, except 

 along their middle and upper surface, which is connected by the 

 large vessels, &c. before mentioned, to the spine. They may be 

 said to completely fill the cavity, their external covering of pleura 

 being in contact with the reflection of the membrane which lines 

 the chest. 



Respiration consists of inspiration and expiration, and the bulk 

 of the lungs will accord with the dilatation or contraction of the 

 cavity; nevertheless they are not, as elsewhere stated, mere 

 passive agents in the process. Many of the muscles which lie 

 externally to the ribs, as well as those filling the spaces between 

 them, the musculo-tendinous partition between the thorax and 

 the abdomen, the diaphragm, and the abdominal muscles, are 

 concerned in breathing, In expiration a portion only of the air 

 contained in the air-cells is forced out by the pressing forwards 

 of the viscera of the abdomen upon the thoracic cavity, through 

 the contraction of the abdominal muscles, the diaphragm being 

 at that time in a relaxed condition ; the sides are also compressed 

 at the same instant by the fall of the ribs, which is aided in part 

 by their cartilages. This action ceasing, the diaphragm contracts, 

 and assumes a flatter aspect ; the viscera of the abdomen recede, 

 and the ribs, the motion being assisted by their synovial joints, 

 are drawn forwards and outwards, thus enlarging the cavity. To 

 fill the vacuum which would thus be occasioned, a rush of fresh 

 atmospheric air down the windpipe takes place: this equalizes 

 the density of that portion of the air which had not been expelled, 

 and which by its retention had become rarefied, and thereby assists 

 the expansion of the lungs ; the pressure to which they were sub- 

 jected in expiration being now removed. As the chief use of this 

 function is to eject carbonic acid gas from the system and produce 

 oxygenated blood, so the quantity of air respired in a given time will 

 be regulated accordingly. In a state of quietude and in health, the 

 number of respirations in the ox are about 1 2 in the minute ; being 

 in proportion of 1 to 4^ of the pulsations. The quantity of carbonic 

 acid evolved varies from four to even eight per cent. ; the rapidity 

 of its production depending, amongst other causes, on the amount 

 of exertion an animal undergoes. To supply the necessary- 

 quantity of oxygen to combine with the carbon, an increase of 

 breathing must take place, otherwise death will quickly ensue. 

 This rapid combustion of the carbon would, however, raise the 

 temperature of the body far too high compatible with the main- 

 tenance of its functions ; and consequently, as the circulation is 

 increased, so will be the secretion from the follicles of the skin, 



