302 



VETERINARY PHYSIOLOGY 



pull downwards the lower ribs, and the triangularis sterni also 

 assists in this. 



By this constriction of the thorax, brought about by ordin- 

 ary or by forced expiration, the air inside is compressed and 

 the pressure raised. During ordinary expiration the highest 

 pressure reached is about 2 to 3 mm. Hg, in forced expiration 

 about 80 mm. in man. 



The pressure of the air outside is less than this, and the air 

 inside the chest is driven out. 



I4,OOO 



14,000 cc 



10,000 c.c 



Copaci/y 



Special Respiratory Movements. There are several peculiar 

 and special re Hex actions of the respiratory muscles, each 

 caused by the stimulation of a special 

 district, and each having a special purpose. 

 Coughing. This consists of an inspira- 

 tion followed by a strong expiratory effort 

 during which the glottis is constricted but 

 is forced open repeatedly by the current 

 of expired air. It is generally due to irrita- 

 tion of the respiratory tract, and its object 

 is to expel foreign matters. 



Sneezing. This is generally produced by 

 irritation of the nasal mucous membrane, 

 and its object is to expel irritating matter. 

 It consists in an inspiratory act followed 

 by a forced expiration during which, by 

 contraction of the pillars of the fauces and descent of the soft 

 palate, the air is forced through the nose. 



Hiccough consists in a sudden reflex contraction of the 

 diaphragm causing a sudden inspiration which is interrupted 

 by a spasmodic contraction of the glottis. Abdominal irrita- 

 tion is its chief cause. It is allied to vomiting. 



Sighing and Yawning are deep involuntary inspirations 

 which serve to accelerate the circulation of the blood when, 

 from any cause, it becomes less active (see p. 291). They are 

 probably due to cerebral anaemia, which they help to correct by 

 increasing the general arterial pressure. 



II. Amount of Air respired. The amount of air respired is 

 different in ordinary and in forced respiration. 



FIG. 138. The amount 

 of Air respired in or- 

 dinary Respiration, 

 and in forced Inspira- 

 tion and Expiration. 



