THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 187 



concealed from view by portions of the larynx. This opening should be located 

 by probing; the probe will be found to enter a soft tube which proceeds poste- 

 riorly dorsal to the larynx. This tube is the esophagus. 



The structure of the larynx should now be examined in detail. The ventral 

 wall of the larynx is supported by a large shield-shaped cartilage, the thyroid 

 cartilage. A short distance posterior to this is the cricoid cartilage, which 

 forms a ring around the larynx. The dorsal rim of the glottis between the glottis 

 and the opening to the esophagus is supported by a pair of projecting cartilages, 

 the arytenoids. On looking into the glottis a pair of folds, the vocal cords, will 

 be seen extending from the arytenoid cartilages to the thyroid cartilage. They 

 nearly occlude the opening. In the cat, in addition to these true vocal cords, 

 there is a pair of false vocal cords, situated lateral to the former and extending 

 from the tips of the arytenoid cartilages to the base of the epiglottis. It will 

 be noted that the vocal cords are not cords but folds of the lateral wall of the 

 larynx. On dissecting away the esophagus from the dorsal side of the larynx 

 the dorsal side of the cricoid cartilage will be exposed. It is much broader than 

 the ventral side. By cleaning away the mucous membrane covering it, the two 

 arytenoid cartilages which rest on the anterior extremity of the dorsal part of 

 the cricoid will be exposed. 



From the larynx the trachea or windpipe proceeds posteriorly. Its walls 

 are stiffened by cartilaginous rings, which are incomplete dorsally, leaving a 

 soft strip in the dorsal wall of the trachea into which the esophagus fits. On 

 each side of the trachea lying against the trachea and internal to the muscles is 

 a flattened elongated body, one of the lobes of the thyroid gland. The anterior 

 end of each lobe is at a level with the cricoid cartilage. The caudal ends of the 

 two lobes are connected by a median portion, the isthmus, which crosses the 

 ventral side of the trachea. The trachea is not to be traced farther posteriorly 

 at this time. 



3. The pleural and pericardial cavities. — The trunk of mammals is divided 

 into an anterior thoracic region and a posterior abdominal region. Each of these 

 regions contains cavities which are portions of the coelom. The thoracic region 

 has three coelomic cavities, the two pleural cavities, laterally located, and the 

 median pericardial cavity, situated between the two pleural cavities. 



With the bone scissors make a cut through the ribs one-half inch to the left 

 of the sternum, extending the cut the length of the sternum. At each end of 

 this, cut laterally and dorsally between two adjacent ribs at right angles to the 

 first cut. In this way a flap is formed in the chest wall. Open the flap and bend 

 it dorsally so. that you can look within. The cavity thus revealed is the left 

 pleural cavity or pleural sac, as it is often called; a similar sac exists on the right 

 side. The pleural sac contains the soft spongy lung. In the median region 

 under the sternum lies the large heart. Note the delicate partition which 

 stretches from the heart to the ventral median line. This partition is called the 



