ORGANS OF RESPIRATION. 567 



without convolutions or enlargements in its course, or an 

 inferior larynx ; it is frequently directed a little to the right 

 of the oesophagus, and is closely surrounded with rings, 

 for the most part incomplete on their posterior aspect, some- 

 times incomplete in front, as in many of the cetacea, and of 

 a cartilaginous texture, though often becoming ossified in 

 advanced age. It divides in the cavity of the thorax com- 

 monly into two, sometimes into three bronchial tubes, which 

 again subdivide before entering the substance of the lungs. 

 The bronchi often continue their cartilaginous rings, with 

 their intervening fibrous structure, through their minuter 

 divisions in the lungs. The posterior vacant spaces between 

 the ends of the tracheal rings, present distinct transverse 

 connecting fibres, and longitudinal fasciculi of fibres are seen 

 passing between the margins of the successive rings. The 

 tracheal rings are completed behind in the most diversified 

 forms of mammalia, as in the long-necked rumiriantia, and 

 the short-necked cetacea, in burrowing rodentia, and in 

 flying chiroptera, and probably relates to the extent of 

 motion or pressure to which this part may be exposed by 

 the living habits of the species. The trachea appears to be 

 somewhat elongated and curved in the thorax of bradypus, 

 as common in the feathered tribes to which they are so 

 much allied. 



The larynx of mammalia, like that of inferior vertebra ta, 

 is formed by the development and consolidation of tracheal 

 rings, and presents the most complex and perfect form of 

 this part, being formed on the anterior and lateral parts by 

 the thyroid cartilage, on the posterior and inferior parts by 

 the cricoid, and on the upper and inner part by the arytee- 

 noid cartilages. The thyroid cartilage, which meets behind 

 in birds as well as in front, is always widely deficient on the 

 back part in mammalia, where the interspace is occupied by 

 the now greatly developed cricoid. The two posterior pieces 

 of the thyroid of birds, as shown by the careful researches of 

 Henle, have here coalesced with the cricoid, to compose a 

 part of that cartilage, as the styl-hyoid bone of inferior 

 quadrupeds separates from the os hyoides, and anchyloses 

 with the temporal in man, to form a process of that bone. 

 The posterior margin of the thyroid is extended downwards 

 on each side, to form the narrow inferior cornu for the 



