RESPIRATORY SYSTEM II5 



on the presence of a number of branchial arches which form 

 parts of the visceral skeleton and are furnished with branchial 

 lamellae. The gradual reduction of the number of branchial 

 arches goes hand in hand with a higher development of the 

 actual branchiae. The presence of a swimming bladder formed 

 from an expansion of the primitive intestinal wall is introductory 

 to the respiration of air and, in the dipnoi, has been transformed 

 into a lung. 



From the amphibians upwards we find pulmonary breathing 

 steadily developing as the sole method of respiration, and con- 

 formably thereto arises a special membranous cartilaginous 

 system of air-passages, including an apparatus for the produc- 

 tion of the voice. The special muscles which serve to set in 

 motion the laryngeal cartilage appear in the reptiles as con- 

 stricting and dilating muscles, reaching a considerably higher 

 development in the birds and the highest perfection in man. 



In some of the amphibians the air-passages situated under 

 the larynx are divided into the trachea and two bronchi ; in 

 the reptiles the cricoid cartilage shows an increasing tendency 

 to meet, while in the birds the rings are clearly separated. 

 The lungs of the amphibians are of very simple construction, 

 resembling those of the dipnoi. But in the reptiles the lungs 

 are subdivided into a number of small compartments, and are 

 thus enabled to effect the increased exchange of gases in- 

 volved by a greater supply of blood. 



In birds the lungs are of more complex construction, but 

 the pulmonary parenchyma is still of a spongy nature, in that 

 the most minute cells are connected with each other. 



In mammals the lobular construction of the lungs reaches 

 a high degree of perfection. Although the respiratory organs 

 of man correspond on the whole to those of the anthropoids 

 they nevertheless differ in many particulars. The larynx of 

 the chimpanzee most closely resembles the human larnyx (see 

 Fig. 63), while that of the orang is least like man's. In the 

 anthropoids the anterior part of the glottis proper is short, not 

 longer than the respiratory part of the glottis. 



One of the most striking differences between the anthro- 

 poids and man is seen in those broad, frequently dilatable skin 



and air sacs, connected with the sinuses of Morgagni, possessed 



s * 



