y^asal Passages of the CiwJialot. 01:5 



bio'S^er — a state oF affairs quite tlic reverse oi; tliat whieh we 

 have just seen to oeciir. A. little later they again heeorne 

 equal, though still remaining parallel and vertical in 

 position with refereuce to the head. This point is 2*1 mui. 

 further on. 



Very soon, indeed, the axis of the two nasal passages, 

 thongh themselves remaining quite equal in size, eome to 

 lie at an angle with each other ; but that angle is exactly 

 the reverse of what it has been, and they bend away from 

 each other at their upper and not at their ioiver ends. 

 There is no further alteration of moment until the two tubes 

 fuse to form one — the nasal pharynx, — which point is at the 

 distance of another 2 mm. 



It will be obvious, of course, that on a lateral view the 

 nasal passages would be seen to slope gradually downwards 

 from their original position on a level with the blow-holes. 

 The various changes in the position of the two nasal canals 

 with reference to each other — i. e. the mutual and con- 

 tinually differing angles — indicate that the left canal, at 

 any rate, is spirally arranged, and, in fact, turns upon 

 itself along the whole length half a turn. The right canal 

 is very nearly the same, but a slight complication is intro- 

 duced by the huge size of its terminal chamber. 



The symmetiy of the two tubes is another matter of 

 interest. As has been pointed out, they are not at first 

 symmetrical with the axis of the head. Later on, howeser, 

 they come to lie pretty exactly one on either side of the 

 vertical median line. This rectification of the asymmetry 

 visible in the anterior part of the head is visible about 

 2'7 ram. from the end of the snout or possibly a trifle 

 earlier. 



A third matter worthy of attention is that the two canals 

 fluctuate in size at more than one place. Each tube, seen 

 foreshortened, would appear moniliform. This change of 

 size, however, is limited to the area in front of the final 

 arrangement of the two tubes at an angle diverging from 

 the ventral side. They are thereafter exactly ot the same 

 size. Nor is their opening into the nasal pharynx associated 

 with any increase or diminution of size in one or the other. 

 They simply fuse» 



§ Comparison of the Nasal Passages in the Foetus with 

 those of the Adult IVltale. 



The chief authorities known to me on the structure of 



