Trachea are generally highly elastic but they may calcify with age. The trachealis muscle connects the ends of the incomplete rings and contracts during coughing, reducing the size of the lumen of the trachea to increase the rate of air flow. The esophagus lies posteriorly to the trachea, adjoining along the tracheoesophageal stripe. Circular horizontal bands of fibrous tissue called the annular ligaments of trachea join the tracheal rings together. The cartilaginous rings are incomplete to allow the trachea to collapse slightly so that food can pass down the esophagus. A flap-like epiglottis closes the opening to the larynx during swallowing
Traditional Chinese medicines derived from the human body were successfully making quasi-empirical preparations of active substances with androgens and estrogens, a technique that modern biochemists did not develop until the early 20th century. The physician William C. Cooper and the sinologist Nathan Sivin (1973) chose what the Chinese call "rényào" 人藥 "human drugs" as a pilot experiment sample for pharmacologically analyzing the efficacy of drugs used in TCM. In contrast to many traditional Chinese plant, animal, and mineral pharmaceuticals with uncertain active constituents, the chemical composition of the human body and parts is well known, and "their therapeutic effectiveness, or lack of it, can be objectively, if approximately,