522 VETERINARY PHYSIOLOGY 



3. Special Respiratory Movements. — There are several peculiar 

 and special reflex actions of the respiratory muscles, each caused 

 by the stimulation of a special region, and each having a special 

 purpose. They are generally protective reflexes in response to 

 nocuous stimuli. 



Coughing. — This consists of an inspiration 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 irritation of the respiratory tract, and 

 its object is to expel products of inflammation or foreign 

 matter. 



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, (a) by contraction of the pillars of 

 the fauces and descent of the soft palate, and (6) by the tip 

 of the tongue being pressed against the hard palate, the 

 air is compressed and finally forced through the nose and 

 mouth. 



Hiccough consists in a sudden reflex contraction of 

 the diaphragm causing a sudden inspiration which is 

 interrupted by a spasmodic contraction of the glottis. 

 It is allied to vomiting. Abdominal irritation is its chief 

 cause. 



Sighing and Yawning are deep involuntary inspirations 

 which serve to accelerate the circulation of the blood in the 

 brain when, from any cause, it becomes less active. They are 

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

 increasing the general arterial pressure, and they are the result 

 of direct chemical stimulation of the respiratory centre rather 

 than reflex actions (p. 527). 



II. Amount of Air Respired. 



The amount of air respired is diff"erent in ordinary and 

 in forced respiration (fig. 211). 



In an ordinary respiration in the horse about 3000 ccms. of 

 air enter and leave the chest. That is called the tidal air- 

 Its amount varies with the size and muscular development 

 of the chest. 



