i8g 4 .] 



THE BRITISH NATURALIST. 



69 



bronchus, before it reaches the lung, breaks up into two trunks, a large 

 and a small, in this resembling the bears, to which a relationship is also 

 indicated by the cranial skeleton of the walrus. 



Whilst, as has been previously pointed out, the lungs are developed 

 from the oesophagus, it must be remembered that in all lung-breathing 

 vertebrates, some part, if not the whole, of the air required for respiration 

 reaches the lungs not through the mouth but through the nostrils. 

 These organs are connected with the olfactory as well as respiratory 

 functions, and in fact primitively, are solely organs of smell. 



Like the organs of sight and hearing, they arise as involutions of the 

 integument, represented in the lowest fishes by a single sac, but in all the 

 higher vertebrates by two, with which are connected nerve filaments from 

 the olfactory lobes of the brain. It would require considerable time and 

 the use of phrases embarassing to any but the trained anatomist, to trace 

 the modifications of the bones of the skull, associated with the evolution 

 of the nostrils of the Dipnoi from the olfactory sac of amphioxus, and we 

 must content ourselves with the bare statement of the fact. It is 

 evident that unless the entrance to the wind pipe from the alimentary 

 canal is sufficiently protected either by something of the nature of a 

 valve or by some modification of its primitive ventral position food would 

 enter the lungs and speedily cause death. In most animals the anterior 

 portion of the wind pipe is developed into the larynx, an organ which 

 plays a most important part in the production of sound, and which com- 

 municates by a relatively small and narrow orifice with the alimentary 

 canal ; the adjacent parts are generally so modified as to reduce to a 

 minimum the risk of food entering the lung. In some animals, however, 

 no air reaches the lungs through the mouth, as portions of the larynx are 

 modified to form a tube which extends across the alimentary canal and 

 is firmly embraced by the soft palate, so that food passes on either side 

 of this tube whilst the air-passages from the nostrils to the wind pipe are 

 made continuous. This condition obtains in young marsupials, which are 

 born in a very immature state and are placed by their mother in contact 

 with her teats. The mouth ot the little marsupial is completely filled by 

 the teat, and it would be more accurate to say that the milk is pumped 

 into it than that it sucks the milk in. It is evident that without the 

 above-named modification of the larynx the animal could not live five 

 minutes after its birth. The same may be said of the cetaceans, in which 

 similar modifications of the larynx occur, and without which it would be 

 impossible for them to obtain their food as they do, swimming open- 

 mouthed in pursuit of their prey. 



Another peculiarity, facilitating respiration, occurs in some cetaceans, 

 and enables them to remain under water for longer periods than are 

 possible for any other lung-breathing animal. Anyone who has watched 

 a porpoise must have noticed that the contour of the front of the head is 



