178 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



anterior portions fused to form the entoglossal cartilage. Posterior to the entoglos- 

 sal cartilage is a median bony piece, the basihyal. Posterior to this is the basi- 

 branchial of the third gill arch. From the point of junction of basihyal and 

 basibranchial projects on each side the long posterior horn of the hyoid, consisting 

 of portions of the third gill arch. On following the posterior horns they will 

 be found to extend toward the ears and to be divided into a proximal longer 

 portion, the ceratobranchial, and a distal shorter rod, the epibranchial. 



The cartilages of the larynx may next be identified. Cut around the laryngeal 

 prominence freeing it so that it can be drawn ventrally. Also free the hyoid 

 apparatus from the ventral surface of the larynx. The larynx is the expanded 

 chamber thus revealed at the top of the trachea and opening into the pharyngeal 

 cavity by way of the glottis. By dissecting in the margins of the glottis on 

 each side, expose a slender, curved, partially ossified arytenoid cartilage. On 

 the ventral side of the larynx note the enlarged triangular cricoid cartilage. 

 Follow this around to the dorsal side where it terminates by much narrowed 

 ends. Between the two dorsal ends of the cricoid cartilage is another median 

 cartilage, the procricoid, which is in contact with the posterior ends of the aryte- 

 noids. The arytenoids are derived from certain of the gill arches while the 

 cricoid and procricoid cartilages are the most anterior modified rings of 

 the trachea. Although the larynx of birds is morphologically the same as the 

 larynx of other vertebrates from which sounds issue, in birds the voice is not 

 produced in the larynx but in another part of the trachea which will be 

 seen later. 



Examine the cartilages of the trachea. They are broad, hard, and bony ven- 

 trally, but narrower, softer, and cartilaginous in composition dorsally. There 

 is consequently a somewhat soft strip along the dorsal side of the trachea which 

 lies against the cervical vertebrae. 



Trace the esophagus posteriorly. Shortly in front of the sternum it widens 

 into an enormous bilobed sac, the crop. Birds swallow their food whole; the 

 food collects in the crop which is capable of great distension and is passed on into 

 the stomach in small quantities. The crop should be carefully loosened on all 

 sides. Not all birds possess a crop. 



3. The anterior air sacs and the pectoral muscles. The respiratory system 

 of birds is the most remarkable among vertebrates. It consists, not only of 

 the lungs, but also of a number of air sacs located among the viscera and of air 

 spaces in the bones. These air sacs and air spaces communicate with the lungs 

 by means of branches of the bronchi. This system not only aids in decreasing 

 the specific gravity of the bird but also insures a more complete exposure of the 

 lung tissue to the air; for the residual air is retained in the air sacs and not in the 

 lungs as in other vertebrates, and the air in the lungs is consequently completely 

 renewed at each inspiration. Owing to the delicacy of the air sacs the student 



