512 MAMMALIA. 



about 12 Q 00 th of an inch. A renal-portal system is always 

 absent, and in no Mammalia, except Echidna, is there an 

 anterior abdominal vein carrying blood to the liver. 



The lymphatic system is provided with numerous lymphatic 

 glands, and its main trunk (ductus thoracicus), which is placed 

 on the left, opens into the left brachiocephalic vein. 



Of the so-called vascular glands the spleen, the thymus, and 

 the thyroid, which is especially developed in the young, are very 

 generally present. 



The body- cavity of mammals is distinguished by the fact that 

 the two anterior horns, which in Amphibia and many Reptilia ex- 

 tendforwards on each side of the pericardium, are cut off from the 

 general body cavity and constitute the closed pleura! cavities. 

 The diaphragm or midriff, a muscular partition with a central 

 tendinous portion, is developed in the tissue placed between 

 the pleural and pericardial cavities and the abdominal cavity. 

 Abdominal pores are never present and the kidney tubes never 

 have nephrostomata. 



The paired lungs (Fig. 268, p) are freely suspended in the 

 thoracic cavity, and are distinguished by the numerous ramifi- 

 cations of the bronchial tubes, the finest branches of which end 

 with conical, funnel-shaped dilations (infundibula), which are 

 provided on their lateral surfaces with swellings. Respiration 

 is mainly effected by the movements of the diaphragm, which 

 forms a complete, usually transversely placed, septum between 

 the thoracic and abdominal cavities : by the contraction of its 

 muscular parts it acts as an inspiratory muscle ; that is, it 

 dilates the thoracic cavity. The elevation and depression of 

 the ribs also have an effect in dilating the thorax. The trachea 

 is, as a rule, straight, without coils, and divides at its lower end 

 into two bronchi leading to the lungs. There may be, in addi- 

 tion, a small accessory (third) bronchus on the right side. The 

 trachea is supported by cartilaginous half-rings which are open 

 dorsally, and only exceptionally by complete rings of cartilage. 

 The first part of the trachea, or larynx, is placed at the lower end 

 of the pharynx, behind the root of the tongue ; it is supported 

 by the posterior horns of the hyoid bone, possesses vocal chords, 

 complicated pieces of cartilage (cricoid, thyroid, and arytenoid 

 cartilages) and muscles, and constitutes a vocal organ. 



In the Cetacea alone is the larynx, which projects in the base 



