RESPIRATOEY SYSTEM 71 



retention of a primitive character. In special cases the 

 form of the syrinx seems to be of not little value as a mark 

 of affinity. The peculiar syrinx of the storks, for example, 

 distinguishes them from their near allies the herons. The 

 stork-like syrinx of Tantalus is one of the many reasons for 

 placing it with that family. The peculiar form of syrinx 

 termed the ' bronchial syrinx ' may seem to some to militate 

 against the value of this organ as a test of affinity ; but, on 

 consideration, the fact that both owls and goatsuckers 

 possess it will not seem extraordinary in view of their other 

 resemblances, while the cuckoos are perhaps not so widely 

 remote from those two families as they have been placed. 

 We may have here a clue to the relationship of these three 

 groups of birds. The complicated (as regards musculature) 

 syrinx of the parrots is so far an indication of affinity with 

 certain Passeres. The systematic position of the parrots is 

 by no means clearly defined, and therefore this indication 

 of a possible affinity must not be ignored. 



The Lungs and Air Sacs. — The lungs of birds never 

 depend freely in the ccelom, as is the case with most 

 reptiles. They are closely fixed to the parietes, and covered 

 with a thin and transparent aponeurosis, which is the 

 peritoneum. So closely are they adpressed to the body 

 walls that when they are carefully removed by dissection an 

 impress of the ribs is to be seen upon their dorsal and 

 lateral surfaces. An approach to this peculiar position of 

 the lungs in Aves is to be seen in the crocodilia and to a 

 less extent in the Chelonia and Monitor lizards. In these 

 animals the lungs are bound down to the parietes, and do 

 not hang freely, as in the Lacertilia generally and in the 

 mammalia. The lungs in birds occupy the space between 

 the first rib in front and the anterior end of the kidney 

 behind. They nearly meet in the middle line. Seen from 

 below, when left undisturbed in the body, only the adjacent 

 structures being cleared away, the lungs present two facets, 

 an anterior and a posterior ; the latter is divided from the 

 former by a ridge which does not divide the lung into two 

 equal halves. The anterior is considerably the smaller. 



