244 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



large quantity of warm air is rapidly expelled through 

 the single spiracle (the homologue of the two external 

 nares of other animals), or so-called "blow-hole," and 

 that the moisture in this air condenses into water 

 as it suddenly comes into a colder medium. A whale 

 no more breathes water than does a man on a frosty 

 day. 



Pulmonary respiration, or the taking-in and the 

 expulsion of air from the lungs, is effected in very 

 various ways in different Vertebrates ; the air tube 

 being, as we know, ordinarily kept open by the car- 

 tilaginous or bony rings or supports which are found 

 in its walls, it is clear that air may either be driven 

 out or sucked in. 



In the Dipnoi there is no cartilaginous trachea, 

 and the air enters in and passes out by a longitudinal 

 slit, the sides of which are separated from one another 

 by the contraction of the muscle that surrounds it. 

 In the Perennibranchiata there is no true trachea, but 

 on either side of the slit there is a small cartilage 

 with which a constrictor or a dilatator muscle is 

 connected (Wiedersheim). In the rest of the Am- 

 phibia there is a true trachea of no great length, the 

 opening into which is sometimes provided with muscles, 

 by means of which it can be enlarged or diminished 

 in size. In the frog, whose physiology has been more 

 fully studied, air is known to be forced into the lungs 

 by the action of the muscles of the floor of the mouth ; 

 this apparatus, appropriately known as the buccal 

 pump, acts in the following manner : the mouth is 

 shut, and the floor of the mouth depressed by the 

 contraction of the muscles connected therewith ; the 

 vacuum so formed is filled by the entrance of air 

 through the nostrils and nasal passages ; the nostrils 

 are then closed, and the entrance to the gullet barred, 

 while the floor of the mouth rises on the contraction of 

 the muscles connected with the hyoid ; the entrance 



