A Laboratory Manual of the Anatomy of the Rat/The Respiratory System
THE RESPIRATORY SYSTEM
The thoracic cavity of mammals is lined with a thin membrane, the pleura, in the form of two pleural sacs, one on either side of the median plane. The costal pleura covers the body wall, the diaphragmatic pleura the anterior surface of the diaphragm, while the pulmonary pleura is reflected over the outer surfaces of the lungs. The two pleural sacs meet in the median plane to form the mediastinal septum. The smooth surfaces of the pleura reduce the friction between the organs of the thorax.
The heart has already been removed. Clear away the muscle on the ventral side of the neck, exposing the trachea (or windpipe) and esophagus. The larynx, to be described later, is at the anterior end of the trachea. The thyroid gland is an H-shaped gland of internal secretion lying on the trachea just back of the larynx. The isthmus, which corresponds to the transverse bar of the H, runs across the ventral surface of the esophagus, joining the two lateral lobes of the gland. Throughout its length the trachea is in contact with the esophagus. The latter is dorsal to the larynx, but gradually swerves to the left until it passes into the thorax on the left side of the trachea. The heart and the vessels connected with it are ventral to both esophagus and trachea. The trachea bifurcates posteriorly, forming the two primary bronchi. Each bronchus leads to a lung. Observe the cartilaginous rings imbedded in the walls of the trachea and bronchi. They keep the lumen open for the free passage of air. Explain why the dorsal wall of the trachea is more flexible than the ventral.
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 its wall — the thyroid, cricoid, and arytenoid cartilages. This may be done more easily under a dissecting microscope. The thyroid cartilage is a band extending around the anteroventral and lateral regions of the larynx. Each lateral extremity gives off an anterior and posterior horn, the latter joining the cricoid cartilage. The ventral portion of the cartilage bears a median notch on the anterior border, and is bound to the cricoid cartilage by the cricothyroid ligament. The cricoid cartilage has the shape of a signet ring, completely encircling the air passage. It is narrow ventrally, but expands dorsally into a hexagonal plate whose dorsal surface is concave. It is firmly united to the trachea by connective tissue. The arytenoids are a pair of small cartilages articulating with the anterior border of the dorsal plate of the cricoid. Each attaches to the dorsal end of a vocal cord.
Exercise XVIII. Draw and label the larynx.