588 CLASS XVIT. 



above the ordinary one on the right side. The right lung is larger 

 than the left, and has often one or two lobes more than it ; some- 

 times the left is even altogether undivided. Both lungs are un- 

 divided in. the Cetacea, the horse, and some few other mammals. 

 Inspiration and expiration are effected by the same muscles as in 

 man. We ought here to direct attention to the diaphragm, a 

 muscular partition between the cavities of the thorax and abdo- 

 men, peculiar to mammals (p. 556), which is attached to the ver- 

 tebrae, the ribs and the sternum, and is tendinous in the middle. 

 When the diaphragm contracts, it descends; hence it is a muscle 

 of inspiration, for when the capacity of the thorax is increased the 

 air rushes into the lungs and the abdominal viscera are compressed. 

 In some mammals a small bone is found in its tissue. The dia- 

 phragm is perforated by the oesophagus, the large artery, the 

 posterior vena cava, different nerves and the thoracic duct. 



Here we may conveniently turn our attention to the vocal 

 apparatus of mammals. The trachea in mammals differs from that 

 of birds in its relation to the production of voice. In mammals it 

 merely conducts the air which is subservient to the formation of the 

 voice; in birds, on the contrary, the sound itself, for in them the 

 vocal organs are placed at its inferior extremity. Hence in mam- 

 mals the voice cannot be produced if the air should find an outlet 

 through a wound in the trachea. The larynx, in which the voice is 

 formed, consists of different cartilages, and is situated at the top of the 

 trachea. The cartilages are on the whole the same as in man. The 

 crico'id cartilage fonns foundation of the entire larynx, and is broader 

 behind, narrower in front ; above it is placed the thyroid cartilage, 

 which consists of two laminae, which meet at the fore part under an 

 obtuse, or sometimes an acute angle. By two descending branches 

 the thyroid is connected with the crico'id cartilage ; it is connected 

 to the tongue-bone by ligaments, which usually arise from its two 

 superior processes or horns. The thyroid cartilage thus forms the 

 anterior part of the larynx. Two arytceno'id cartilages are fixed on 

 the opposite side to the upper margin of the crico'id cartilage. (In 

 many mammals two roundish cartilages are placed upon the points 

 of these, the cartilagines Santoriance. Between the arytsenoid car- 

 tilages and the epiglottis there are often small cartilages named the 

 Wrisbergian}. The epiglottis is a tongue-shaped cartilaginous 

 lamina, which is attached to the inside of the upper margin of 

 the thyroid cartilage. The larynx communicates with the cavity 



