572 ANATOMY OF VERTEBRATES. 



CHAPTER XXXIII. 



RESPIRATORY SYSTEM OF MAMMALIA. 



(354. Lungs of Mammalia. — The class-characteristic afforded 

 by these organs is defined in vol. ii. p. 266, and exemplified in 

 fig. 139, ib. In all Mammals each lung, ib. lg, is conical, with the 

 base resting upon the diaphragm, ib. d, and the apex reaching to 

 the root of the neck : the shape, and especially the degree of sub- 

 division, of the pulmonary cone offer many varieties in the class. 

 The most common quadrupedal difference from the bimanal type 

 is the lobe, called ' azygos ' or ' impar,' detached from the right 

 lung to occupy the space between the heart and diaphragm, as at 

 n, fig. 308 (Ornithorhgnchus). The outer surface of the lung 

 is smooth, being covered by a serous membrane, reflected from 

 the great blood- and air-vessels forming its ( root ' upon the walls 

 of the thorax ; thus constituting a shut sac, called ( pleural,' 

 distinct from that of the opposite lung. The portions of the 

 pleurae passing respectively from the pulmonary roots to the back 

 and fore parts of the thoracic cavity, are called l mediastinal,' and 

 intercept the pericardium, great vessels, thymus, gullet, and other 

 parts intervening between the two lungs: the regions of such 

 thoracic septum being defined, in Anthropotomy, as e anterior,' 

 ' posterior,' and ' middle mediastinal The pleural serous sacs are 

 peculiar to Mammalia : they facilitate the movements of the lung 

 upon the thoracic walls during respiration. 



The wind-pipe bifurcates to supply each lung, fig. 418, p, p*, 

 with air, as does the pulmonary artery conveying the blood 

 to be affected thereby ; the pulmonary veins, ib. p, return the 

 blood so changed to the heart. Besides these three main con- 

 stituents of the f root ' of the lung, it includes the ' bronchial ' or 

 nutritive arteries and veins, absorbents and nerves, with their 

 connective tissue, and the enveloping pleural sheath. Beneath 

 the serous covering of the lung is a layer of combined areolar and 

 elastic tissues, the latter predominating in the denser c sub-serous' 

 coat of the lungs of the larger carnivorous and ungulate mammals : 

 in Cetacea the smooth contractile fibre is therewith intermixed. 



The trachea is kept patent by cartilaginous hoops, the ends 



