246 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



well as from the outer world ; and when the thorax 

 contracts, in the act of expiration, air is driven into 

 the air sacs as well as through the trachea outwards. 



In Mammals the movements of the ribs are 

 greatly aided by the flattening out, or curving up- 

 wards, of the diaphragm, or muscular partition 

 which separates the thoracic from the abdominal 

 portion of the body cavity. The respiratory move- 

 ments of mammals have been fully studied in Man. 

 (See " Human Physiology," chap, v.) 



Bert has collected a large number of statistics 

 with regard to the number of respiratory move- 

 ments executed per minute by various animals. 

 From this we learn that, on the whole, they are 

 more numerous in Mammals than in Birds. A rat, 

 for example, has been seen to make 320 movements 

 a minute, while the canary gives the highest (100) 

 number for birds. Rodents generally respire fre- 

 quently ; the dog and ox 15 times, the lion and horse 

 10, and a hippopotamus was on one occasion observed 

 to breathe only once in a minute ; some large birds, 

 such as ' the marabou, pelican, or condor, only 4 to 6 

 times a minute; a Crotalus 5 times, a lizard 12. An 

 active sea- lamprey gave a number of 120 ; rays and 

 dog-fishes from 40 to 50, Limulus 12, while Cepha- 

 lopods varied between 14 and 65 times a minute. On 

 the whole, carnivorous breathe less frequently than 

 herbivorous forms, and both than rodents ; smaller 

 forms more frequently than larger members of the 

 same group, and active more often than sluggish 

 species. It is, however, to be carefully observed 

 that these numbers give us no information as to the 

 quantity of air taken in, nor as to the number of 

 times in which the heart was beating per minute; 



