CHAPTER XVII. 



THE RESPIRATORY SYSTEM. 



Introductory. — In the 3rd week the arrangement of - the 

 heart and branchial arches in the human embryo is that of a 

 water -breathing animal. In water -breathing animals the 

 tubular heart expends its force in pumping the blood through 

 the visceral arches — 'necessarily weakening the circulation in 

 the rest of the body beyond them (Figs. 2 1 A and B). The 

 blood that returns to the heart and from which the heart 

 is nourished is purely venous. In amphibians an air- 

 breathing apparatus appears — hence the division in them of 

 the auricular chamber, one chamber receiving systemic, the 

 other pulmonary blood. In amphibians the air is pumped into 

 the lung by the mouth and pharynx. In birds and mammals 

 the division of the heart into pulmonary and systematic pumps 

 becomes complete ; the chest wall is developed as a respiratory 

 apparatus. The pharynx may be regarded as the respiratory 

 organ of water breathers, and it is from the floor of this chamber 

 that the pulmonary or air-breathing structures are developed. 

 The arrangement of the heart and branchial arches in the human 

 embryo can be explained only on the supposition that the air- 

 breathing animals are the descendants of a water-breathing 

 stock. The addition of the pulmonary system commences in 

 the human embryo at the 3rd week. 



Development of the Pulmonary System.— In the 3rd week, 

 towards the end of it, a deep groove appears in the floor of 

 the primitive pharynx and oesophagus. The groove or trough- 

 like depression of the fore-gut commences between the ventricle 

 ends of the 4th arch and stretches almost to the stomach 

 (Fig. 204). The furcula, formed from the ventral parts of the 



