and the left lung into two ; while in the coiv, the right is divided into 

 four lobes, and the left into two. The inter-lobular tissue is also found 

 in greater abundance in the cow than in the horse. The nutrient 

 blood-vessels 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 artevies 

 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. 405). 



480. The 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. 



481. The Pleura is a fine, serous membrane, which lines the inside 

 of the ribs on both sides (pleura costalisj, 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 mediastinum, 

 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. 



482. 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 — 

 (i) 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. 405). 

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

 lungs, charged with carbonic acid gas and other impurites, 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 " 



