Trachea act as a resonator to amplify sound. In some birds, the tracheal rings are complete, and may even be ossified. In amphibians, the trachea is normally extremely short, and leads directly into the lungs, without clear primary bronchi. A longer trachea is, however, found in some long-necked salamanders, and in caecilians. While there are irregular cartilagenous nodules on the amphibian trachea, these do not form the rings found in amniotes. The only vertebrates to have lungs, but no trachea, are the lungfish and the "Polypterus", in which the lungs arise directly from the pharynx. The word "trachea" is used to
Composition of the human body g in men, ~2.3 g in women Of the 94 naturally occurring chemical elements, 60 are listed in the table above. Of the remaining 34, it is not known how many occur in the human body. Most of the elements needed for life are relatively common in the Earth's crust. Aluminium, the third most common element in the Earth's crust (after oxygen and silicon), serves no function in living cells, but is harmful in large amounts. Transferrins can bind aluminium. The composition of the human body is expressed in terms of chemicals: The composition of the human body can be