280 ESSENTIALS OF ZOOLOGY 



The windpipe at its anterior end is modified into the larynx. 

 The upper horseshoe-shaped cartilage is the thyroid, and it 

 supports the epiglottis. The lower circular cartilage is the 

 cricoid. The latter supports on its upper margin two small 

 curved cartilages associated with other two small nodules. 

 These are respectively the arytenoid cartilages, the cartilages 

 of Wrisberg, and the cartilages of Santorini. The arytenoids 

 support the dorsal lips of two folds of the mucous membrane 

 which stretch from the thyroid to them. These are the vocal 

 cords, and the aperture between them may be contracted or 

 expanded by muscles acting upon the arytenoids. The 

 larynx is the organ of the voice. Below it, and embracing 

 the trachea, the thyroid gland will be seen as a red organ 

 expanding on each side into a lobe. 



The trachea passes along the neck into the thorax. Its 

 walls are supported by horseshoe-shaped cartilages which keep 

 it distended and elastic. The trachea branches into the two 

 bronchial tubes. In the lung each of these is divided into 

 tubes gradually becoming smaller in diameter. The small 

 branches are termed bronchioles, and they end finally in 

 slightly expanded chambers called alveoli. The cartilages 

 distend the tubes for some distance into the lungs, but they 

 gradually disappear and the walls become thin. In the 

 alveoli the wall is a thin flat epithelium which brings the air 

 into intimate contact with the blood in the capillaries which 

 ramify around them. The oxygen is taken up in exchange for 

 gaseous materials of a waste character, but mainly for carbon 

 dioxide. The oxygen is carried away by the blood loosely 

 united with the respiratory pigment, haemoglobin, lodged in 

 the erythrocytes. It is carried to all parts of the body by the 

 circulation of the blood, and is brought by the capillaries 

 into contact with the living cells doing work at the expense of 

 the energy liberated by the oxidations of fats and carbo- 

 hydrates brought also by the blood to them. 



There is no air in the lungs of the foetus, but at birth the 

 lungs are distended with air. The air contents of the lungs 

 are increased and decreased by the expansion and contraction 

 of the thoracic cavity. The ribs are rotated forwards by the 

 intercostal muscles, and the diaphragm flattened backwards by 



