THE ANATOMY OF THE WEDDELL SEAL. 77 



than that of KEITH,* who approaches the discussion of the subject more as an anatomist 

 than as a physiologist. 



It is not my intention to follow Professor KEITH in detail and offer a criticism of the 

 conclusions arrived at by him. At the same time, some of his statements appear to me 

 to overlook certain of the anatomical facts. Perhaps the most important fundamental 

 statement made by KEITH is in reference to the lungs when he says " the upper lobe is 

 normally expanded by one mechanism, the lower by another," and as a consequence he 

 insists that " the great fissure, which divides the upper from the lower lobe, is functional 

 in its significance." Supposing this view to be correct, it would follow that since there 

 is a third lobe in the right lung of man and a fourth or azygos lobe in the right lung of 

 a quadruped, with the fissures required for their delimitation, the mechanism for expand- 

 ing the right lung must differ from that required for the left lung. Further, as regards 

 the apical or upper lobe, KEITH maintains that because of the impressions of certain ribs 

 upon the lateral and anterior aspects of the upper lobe, but not upon " the dorsal surface 

 of the upper lobe," there is " a constant relationship between ribs and spaces " for that 

 part of the lobe which presents impressions, but a " downward and upward " movement 

 of the dorsal unmarked part, in which it follows the movement of the lower lobe, because 

 " the lower lobe and the dorsal part of the upper lobe are chiefly expanded by a dia- 

 phragmatic mechanism." The argument for a functional significance for the great oblique 

 fissure seems to me unnecessary if the substance of the apical lobe is to expand in two 

 different ways simultaneously, for, at least as far as the dorsal part is concerned, the 

 presence of the fissure does not seem to confer any advantage. 



I am not disposed to maintain that the fissures of the lungs have no significance, 

 although to my mind it is rather structural than functional. Even " the obliteration 

 of the pleura! cavity by adhesions has so little apparent effect on the respiratory 

 movements that their presence cannot be detected during life," any more than the 

 obliteration of the lobulated character of the kidneys interferes with their functions. 

 After all, the outstanding requirement is that the lungs shall expand to the capacity 

 corresponding to the immediate muscular effort that is being performed, and naturally, 

 therefore, the capacity undergoes constant variation. With this end in view, I cannot 

 but think it is best to consider the muscular mechanism of inspiration as a whole, and 

 the muscular mechanism of expiration as a whole, since it is their co-ordinated and not 

 their individual action that we depend upon. Probably, in quiet ordinary breathing, 

 no animal, any more than the human individual, employs the full scope of its inspiratory 

 mechanism, and hence in man it has become customary to employ such terms as 

 " thoracic " and " abdominal " to indicate the character of the inspiratory effort which 

 is most noticeable in the female and in the male respectively. At the same time, there 

 is no record of this distinction in the inspiratory act among the sexes of the lower 

 animals, nor between the human sexes during infancy and early adolescence. It 



* "The Mechanism of Respiration in Man," by ARTHUR KEITH, pp. 182-207, in Further Advances in Physiology, 

 edited by LEONARD HILL, published by Edward Arnold, London, 1909. 



(ROY. soc. EDIN. TRANS., VOL. XLVIH., 327.) 



