IV. VERTEBRATA: MAMMALL\ 555 



Most important of respiratory peculiarities is tlie diaphragm, which 

 separates the Ijody cavity into thoracic and abdominal cavities. This 

 occurs only in its beginnings in other vertebrates (perhaps even in Am- 

 phibia). In the thoracic cavity are the oesophagus, heart wth its pericar- 

 dium, and especially the trachea, bronchi, and lungs; the remaining 

 vegetative organs are in the abdominal cavity. The diaphragm is a 

 muscular dome, its convex side towards the thoracic cavity; by contraction 

 it flattens and increases the size of the cavity, in consequence of which air is 

 drawn into the lungs (inspiration). On relaxation the lungs contract from 

 their own elasticity and force out a part of the air (expiration). The in- 

 tercostal muscles, which raise and lower the framework of the chest, also 

 play a part, as in Ijirds. The respiratory ducts begin with the larynx 

 (with vocal cords), supported by the thyreoid cartilage occurring only in 

 mammals, which can be closed from the pharynx by the epiglotlis; this is 

 followed by the trachea, which divides into right and left bronchi. Each 

 bronchus divides again and again, and the iuiest of these divisions, the 

 bronchioles, are continued as alveolar duels to small chambers, the injimdib- 

 ula, both these and the alveolar ducts being lined with small respiratory 

 pockets, the alveoli. (See p. 482.) 



The heart, with two auricles and two ventricles, is completely separated 

 into systemic and pulmonary halves. In early embryonic life the arterial 

 trunk, which at first is simple, is divided into a pulmonary artery, arising 

 from the right half of the heart and carrying venous Ijlood, and an aorta 

 ascendens, with arterial blood, connected with the left half. In contrast 

 with reptiles and birds, the right aortic arch is lost, the left persisting. 



The urogenital system is of great importance in the separation of the 

 group into smaller divisions. In both sexes this consists of practically the 

 same parts in early embryonic life (fig. 602). These are the early formed 

 Wolffian body (11); the permanent kidneys, which appear later and are 

 not shown in the diagram; the urinary bladder (4), a part of the allantois 

 which extends (5) into the fcetal appendages; the three ducts, the jMiil- 

 lerian {m), the Wolffian {iv), and the ureter (3). These ducts no longer 

 empty into the intestine, but into the allantoic structures, the ureters, ex- 

 cept in monotremes, into the base of the urinary bladder, the Wolffian and 

 Miillerian ducts into the urogenital sinus (tig), the lower continuation of 

 the Ijladder. In the anterior wall of the urogenital sinus is a mass of 

 highly vascular tissue {cp), from which and a surrounding fold the exter- 

 nal genitalia are developed. Since the urogenital sinus opens from in 

 front into the intestine, there is always a cloaca {cl) in the emljryonic 

 stages. The entodermal section of the cloaca is separated in all mammals 

 into rectum and urogenital sinus. In the monotremes there is added a deep 



