THE RESPIRATORY ORGANS 307 



bloodvessels of the lungs are the bronchial arteries and veins. 

 The functional vessels are the pulmonary arteries and veins, which 

 are much larger than the nutrient vessels. The pulmonary arteries 

 convey the blood from the right side of the heart to the lungs to be 

 purified, while the pulmonary veins return it to the left side of the 

 heart, as described in the lecture on ' Circulation ' (par. 407). 



484. Thorax, or Chest, has a part of the back-bone for a roof, 

 the breast-bone, or sternum, for a floor, the ribs and muscles for 

 lateral walls. The diaphragm is a strong musculo-membranous 

 partition, which separates the chest from the belly, or abdominal 

 cavity. The inside of the thorax is lined by the pleura. 



485. Pleura is a fine, serous membrane, which lines the inside of 

 the ribs on both sides {pleura costalis), and covers the anterior surface 

 of the diaphragm. From the top of the chest and under side of the 

 back-bone, it descends through the middle of the chest to the sternum, 

 dividing the thorax into two lateral halves and forming the medias- 

 tinum, which encloses and covers the heart. The pleura is also 

 reflected over the outside of the lungs themselves, and is then called 

 the pleura pulmonalis. 



486. Respiration, or breathing, is the act by which a constant 

 interchange of gases takes place between the atmosphere and the 

 blood. Two distinct movements are noticed during respiration — 

 (1) inspiration, and (2) expiration. Inspiration is the act by which 

 the lungs become filled with air, for the purpose of purifying the 

 blood, as described in the lecture on ' Circulation ' (par. 407). 

 Expiration, on the other hand, is the act whereby the air in the 

 lungs, charged with carbonic acid gas and other impurities, is expelled 

 from the body. Each inspiration occupies about thrice the length of 

 time taken up by an expiration. The air which passes to and fro 

 during ordinary respiration is called the ' tidal air '; the ' reserve air ' 

 is that which can be voluntarily ejected after ordinary expiration ; 

 ' -complemented air' is that which can be taken in after ordinary inspira- 

 tion ; and ' residual air ' is that which remains after forced expiration. 

 Horses in large towns suffer more from derangement of the respira- 

 tory organs than those in the country, and are considerably worse to 



