THE TRACHEA AND BRONCHI. 477 



at the base, and passing downwards in the ventricular wall, enter the lower 

 part of the septum, in which they ascend to the central fibro-cartilage. 



d. Similar looped fibres pass from the outer wall of the right ventricle and of right 

 through the septum to the fibro-cartilage. ventricle; 



e. The figure-of-8 fibres pass from the front of the right ventricle through figure-of-8 

 the septum to the back of the left, and from the front of the left to the back fibres - 



of the right, the two sets decussating in the septum. 



Endocardium. Lining the interior of the cavities of the heart is a thin Lining 

 membrane, which is named endocardium. ID is continuous on the one hand 

 with the lining of the veins, and on the other with that of the arteries. 

 Where the membrane passes from an auricle to a ventricle, or from a 

 ventricle to an artery, it forms duplicatures in which fibrous tissue is 

 enclosed, thus giving rise to the valves ; and in the ventricles it covers the 

 tendinous cords, and the projecting muscular bundles. The thickness of the 

 membrane is greater in the auricles than in the ventricles, and in the left 

 than in the right half of the heart. 



THE TRACHEA AND LUNGS. 



Dissection. To see fully the pieces of the air-tube in the root 

 of the lung, it will be necessary to divide the branches of the 

 pulmonary artery and the pulmonary veins. And when the upper 

 part of the arch of the aorta is turned to one side, the dissector will 

 be able to clear away the bronchial glands, the nerves, and the 

 connective tissue from the part of the trachea in the thorax, and 

 from the branches into which it bifurcates. 



The TRACHEA, or windpipe, reaches from the larynx to the Trachea 

 lungs, and lies on the front of the spinal column. The tube begins 

 opposite the sixth cervical vertebra ; and it ends commonly at the ends in 

 lower border of the fourth dorsal vertebra by dividing into two 

 pieces (bronchi), one for each lung. 



In the thorax (fig. 163, p. 448) the trachea is situate with the its relations 

 great vessels in the superior mediastinum ; and its lower end is JoSx. 

 usually inclined somewhat to the right side. Here it is covered by 

 the left innominate vein, by the arch of the aorta, with the origins 

 of the innominate and left common carotid arteries, and by the 

 cardiac plexus of nerves. Behind the airtube is the oesophagus, 

 which projects to the left above the arch of the aorta. On the 

 right side are the pleura, the vagus, and the innominate artery for 

 a short distance, after this has passed over the trachea ; and on the 

 left side lie the left subclavian artery, and the recurrent branch of 

 the vagus. 



The BRONCHI, or the branches of the airtube, are contained in the Bronchi lie 

 roots of the lungs, and are surrounded by vessels, glands and nerves. O f the lungs; 

 Xear the lung each is divided into as many primary pieces as there 

 are lobes. In their structure and form the bronchi resemble the are like the 

 windpipe, for they are round and cartilaginous in front, but flat, f[;]* ea m 

 and muscular and membranous behind. Their position behind the 

 other pulmonary vessels has been described at p. 449. 



The right bronchus is about an inch in length, and is larger than The right 

 the left ; it also forms a more direct continuation of the trachea, 

 from which circumstance a foreign body in the airtube is more 

 likely to enter this bronchus. It passes obliquely outwards, on a 



