Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/78

64 The left lung has but one lobe, the right has four. The lobe of the left lung is larger than any of the lobes of the right. Of the lobes of the right lung, three are in a row on the right side of the thorax. Each of these has a convex lateral and a concave median surface. The fourth, and hindmost, lobe of the right lung is approximately pyramidal in shape. The concave base of the pyramid lies against the diaphragm, the apex is directed toward the heart. It lies for the most part at the left of the median plane, and may therefore be mistaken for a lobe of the left lung if its connection with the right bronchus is not disclosed. It is inclosed in the membranous sac previously mentioned.

Exercise XVII. Outline the ventral view of that part of the dissected animal anterior to the diaphragm, and into this outline draw the pleural cavity lungs, esophagus, trachea, thyroid gland, and larynx.

Trace the left primary bronchus into the left lung. Note how it divides into the secondary bronchi, these into the tertiary bronchi, etc. Follow the subdivisions of the right primary bronchus into the lobes of the right lung.

The volume of the thoracic cavity is increased by the periodic contraction of the muscles of the diaphragm and thoracic wall. Atmospheric pressure then forces air into the lungs through the nasal passages, trachea, and bronchi.

Remove the larynx. In doing so injure the pharynx as little as possible. The epiglottis is the flap attached to the anteroventral border of the larynx. When food is being swallowed the epiglottis folds back over the entrance into the larynx, thus preventing the passage of food into the trachea. The vocal cords are the pair of dorsoventral folds in the larynx. The glottis is the slit between them leading to the trachea. Dissect away the shreds of muscle tissue adhering to the larynx, exposing the cartilages in