110 Mammalia, Respiratory System. 



season : Gibbons (Hylobates) alone of brute Mammals may 

 be said to sing. 



The Trachea is kept open by cartilaginous hoops, wbich 

 may be complete or cleft : when cleft, tbe ends of each 

 ring may meet and touch, or overlap, or (as is commonly 

 the case) may be separated even to the extent of a quarter 

 of the circle, the intervening space being occupied by mem- 

 brane : this separation very rarely occurs on the anterior sur- 

 face. The cartilages of the rings may be continued spirally 

 into one another ; and the rings themselves may be occasion- 

 ally found ossified. The number of rings varies, from three 

 (Dugong) to a hundred (Camel). The entire tube is lined 

 by a mucous membrane with a ciliated free surface. The 

 trachea as a rule passes with a straight course until its sub- 

 division into bronchial tubes. As a rule there are two 

 Bronchi, but there is occasionally found a third smaller 

 bronchial tube which passes to the right lung. The bronchi 

 when within the substance of the lung divide and gradually 

 lose their tracheal structure ; the cilia ceasing when the 

 divided bronchial tubes again subdivide to form the numerous 

 'intercellular' passages : the intercellular passages intercom- 

 municate, which the bronchial ramifications never do. The 

 arborescent subdivisions of the intercellular passages terminate 

 in air cells, in size from \ to ^^ of a line : on the parietes of 

 these cells the pulmonary capillaries offer only one side to the 

 respiratory medium, instead of being wholly immersed in the 

 extra-bronchial air as in Birds. Air sacs in connection with 

 the lungs, as are observed in Birds, are not present in 

 Mammalia. 



The Lungs, minutely cellular throughout, are suspended 

 freely in a thoracic cavity, separated by a musculo-tendinous 

 partition or ' diaphragm ' from the abdomen. A complete 

 Diaphragm is peculiar to Mammalia; it is attached to the 

 vertebra?, the ribs, and the sternum, and is tendinous in the 



