THE RESPIRATORY SYSTEM, 



375 



ing ordinary quiet breathing is undetermiued ; if at all, prob- 

 ably but slightly. Though the removal of the external inter- 

 costals in the dog and some other animals reveals the fact 

 that the internal intercostals contract alternately -with the dia- 



VI VII Vlll IX X 



Fig. 300.— Diagram of scorpion, most of the appendages having been removed (after 

 Huxley), a, mouth; d, alimentary tract; c, anus; d, heart; e, pnlmonary sac; /", 

 position of ventral ganglionated cord; g, cerebral ganglia; 7\ telson. VII— XX, 

 seventh to twentieth somite. IV, V, VI, basal joints of pedipalpi and two fol- 

 lowing pairs of limbs. 



phragm, it must not be regarded as absolutely certain that such 

 is their action when their companion muscles are present, for 

 Nature has more ways than one of accomplishing the same 

 purpose — a fact that seems often to be forgotten in reasoning 

 from experiments. This result, however, carries some weight 

 with it. 



Types of Biespiiation. — There are among mammals two prin- 

 cipal types of breathing recognizable — the costal (thoracic) and 

 abdominal — according as the movements of the chest or the 

 abdomen (diaphragm) are the more pronounced. 



Personal Observation.— The student would do well at this 

 stage to test the statements we have made in regard to the respira- 

 tory movements on the human subject especially. This he can 

 very well do in his own person when stripped to the waist be- 

 fore a mirror. Many of the abnormalities of the forced respira- 

 ation of disease may be imitated — in fact, this is one of the 

 departments of physiology in which the human aspects may be 



