Respiratory System General. Ill 



middle ; occasionally a small bone is found in its tissue. Tlie 

 serous membrane covering each, lung is reflected from the root 

 of the lung upon the walls of the thorax, thus constituting a 

 so-called * pleural ' sac. The pleural serous sacs are peculiar to 

 Mammalia. The right lobe of the lung is usually the larger, 

 and is very commonly subdivided : the left lobe is frequently 

 undivided, and when divided has generally fewer lobes than 

 the right. The lungs may be simple and undivided, as in 

 Man, but exceptionally. The most common Quadrupedal dif- 

 ference from the Bimanal type is the lobe called * azygos,* 

 or ' impar,' detached from the right lung to occupy the space 

 between the heart and the diaphragm. 



EESPIRATOEY SYSTEM Special. 



1. The Thyroid. 



Quadrumana. In most Platyrrhina the upper border of the 

 Thyroid is emarginate. In Mycetes (Howler) the Thyroid is twice 

 the size of that of Man, and has a strong anterior prominence, bulg- 

 ing out there to lodge a pair of sacculi continued from the fore part 

 of the long interchordal cleft or ventricle. In Stenops (LemuridaB) 

 the lower cornua of the Thyroid are produced over and beyond the 

 Cricoid to be connected with the first tracheal ring. 



Artiodactyla. In the Hog there are no upper cornua to the 

 Thyroid ; and in the Camelidse the upper cornua are represented only 

 by tubercles. 



Sirenia. The cartilaginous wings of the Thyroid are not conflu- 

 ent, but are joined anteriorly for a short way by sclerous tissue, and 

 below this by membrane and areolar tissue. 



Monotremata. In Ornithorhyncus the lateral alse of the Thyroid 

 are bony, and each of them bifurcated : one of the processes extend- 

 ing to the posterior part of Pharynx, where it becomes cartilaginous, 

 and is confluent with the corresponding process of the opposite side. 



