Cervical titbfi in Man. 101 



displaces the lipfhter luiif; upward ; this sinkinj; is eAMdent from the 

 fact that tlie ])ericardiuiii in man has a more extensive attachment to 

 the diaphraiiiii than it lias in any otlicr animal, and also by the aorta 

 havinof a l(in<j:er ari'h. 



Arthur Keith, in liis work on "" The Mechanism of Kespiratinn in 

 Man," on puj^e 1S7 of " Further Advance^ in Physiology," edited by 

 Leonard Hill, 1909, draws attention to the observation of Colbeck that 

 during inspiration the apices of the lungs recede from the neck. In a 

 rough way this observation can be confirmed by making forced in- 

 spiration in the front of a mirror and there watching the dimpling 

 of skin in the neck that takes place with each inspiration. On page 

 "200 of the same book Keith refers to the observation of Wenckebach, 

 ■ In suljjects of extreme visceroptosis, the diaphragm is thrown out 

 of action bj- its visceral fulcrum being lost, and breathing is carried on 

 by an elevation to the upper part of the thorax." This important 

 observation shows as plainly a« does the effect of tight corset lacing, 

 that curtailment of the acticm of the diaphragm is compensated by 

 costal breathing, and there can be no doubt that the diaphragm is a 

 more efficient muscle of inspiration than the upper intercostal muscles. 

 As the encroachment of the lungs into the neck is the cause of the 

 development of cervical ribs, it follows that full curvature of the upper 

 ribs and loss of the curves of the neck are changes that prepare the 

 way for the apices of the lungs to pass through the space between the 

 tirst ribs, and consequently such changes are co-operating factors in 

 the production of cervical ribs. 



In man the costal development that is associated with the seventh 

 cervical vertebra ranges through all grades from the quadrupedal type 

 to fully formed cervical ribs. In other words, the seventh cervical 

 vertebra of man may show : — 



(1) The absence of costal processes (extinct ribs). 



(2) Costal processes (vestigial ribs). 



(3) Cervical ribs in various stages of development. 



Far reaching as these changes are, they do not include all varia- 

 tions, for unequal development of the two sides of the seventh cervical 

 vertebra is by no means uncommon. 



The quadrupedal baboon is, like other quadi'upeds, devoid of costal 

 processes, and therefore it is probable that during the stage of qiuid- 

 rupedal progression of the human race the seventh vertebra of man 

 was constantly without costal processes ; in other words, these ribs 

 were extinct. 



The gorillas in the Melbourne Museum are without costal processes 

 on the seventh vertebra. An orang, in the same museum, is also 

 without them. One chimpanzee is without costal processes, another 

 has them. Of the skeletons of apes in the Melbourne Museum only 



