RESPIRATION 



Ex. 



occurs in the nasal chambers. In the majority of animals air 

 may pass either through the nose or mouth to enter the trachea, 

 but in the horse, owing to the length of the soft palate, nasal 

 respiration alone is possible ; we therefore find in this animal 

 the nasal chambers with their inlets and outlets well developed. 

 The opening into the nostrils of the horse is large, funnel-shaped, 

 and capable of considerable dilatation ; it is partly cartilaginous 

 and partly muscular. Immediately inside the nostril is a large 

 blind sac, termed the false nostril, and one of its uses appears 

 to be to increase the capacity of the nasal opening by allowing 

 considerable and rapid dilatation. Another use is in the pro- 

 duction of the peculiar snorting sound made by a horse either 

 when he is alarmed or veiy ' fresh.' 



During forced inspiration the nostril expands, especially the 

 outer segment — viz., that part in communication with the false 

 nostril — and the air is 

 rapidly drawn up through 



the nasal chambers ; during v 



expiration the outer segment 

 of the nostril collapses, but 

 the inner segment, composed 

 principally of the cartilagi- 

 nous ala, dilates. Thus the 

 movement of the outer and 

 upper part of the nostril is 

 principally inspiratory, of 

 the lower and inner part 

 mainly expiratory, pro- 

 ducing a peculiar double 

 motion of the nostrils well 

 seen after a gallop or in 

 acute pneumonia (Fig. 47). The dilatation of the inner segment 

 of the nostril is brought about by muscular contraction and 

 by the rush of expired air ; striking the cartilaginous wing of the 

 nostril, the current is directed outwards at an obtuse angle to its 

 course down the nostrils, as may be well seen on a frosty morning 

 when a horse is respiring rapidly. 



The nasal chambers are remarkable for their great depth and 

 narrowness ; the cavities are partly filled by the turbinated 

 bones, which nearly touch the septum on each side, so that a 

 deep but thin column of air passes through the chambers ; the 

 result of this arrangement insures that the air is saturated with 

 vapour and raised to the proper temperature. The nasal 

 chamber is divided into two parts, the lower or respiratory and 

 the upper or olfactory. The latter will be dealt with under 

 the Senses. It comprises the upper portion of the superior 



Fig. 47. 



In. 



Nostril of Horse. 



Ex, the ex- 



The inspiratory portion ; 

 piratory portion 



