144 FUNCTIONS OF THE NERVOUS SYSTEM. 



ner, in cases of palsy, in which the patient cannot raise his shoulder by an 

 effort of the will, but does so in the act of yawning. Nevertheless this act 

 may be performed by the will, though not completely ; and it is one that is 

 particularly excited by an involuntary tendency to imitation ; as every one 

 must have experienced who has ever been in company with a set of yawners. 

 Sobbing is the consequence of a series of short convulsive contractions of the 

 diaphragm ; and it is usually accompanied by a closure of the glottis, so that 

 no air really enters. In Hiccup, the same convulsive inspiratory movement 

 occurs ; and the glottis closes suddenly in the midst of it ; the sound is occa- 

 sioned by the impulse of the column of air in motion against the glottis. In 

 Laughing, a precisely reverse action takes place ; the muscles of expiration 

 are in convulsive movement, more or less violent, and send out the breath in 

 a series of jerks, the glottis being open. This sometimes goes on, until the 

 diaphragm is more arched, and the chest is more completely emptied of air, 

 than it could be by an ordinary movement of expiration. The act of Crying, 

 though occasioned by a contrary emotion, is, so far as the respiration is con- 

 cerned, very nearly the same as the last. Every one knows the effect of 

 mixed emotions, in producing an expression of them which is " between a 

 laugh and a cry." The purposes of the acts of coughing and sneezing are, 

 in both instances, to expel substances from the air-passages which are sources 

 of irritation there ; and this is accomplished in both, by a violent expiratory 

 effort, which sends forth a blast of air from the lungs. Coughing occurs, 

 when the source of irritation is situated at the back of the mouth, in the 

 trachea, or bronchial tubes. The irritation may be produced by acrid vapours, 

 or by liquids or solids, that have found their way into these passages ; or by 

 secretions which have been poured into them in unusual quantity, as the 

 result of disease ; or by the simple entrance of air (especially if cold), when 

 the membrane is in a peculiarly irritable state. Any of these causes may 

 produce an impression upon the excitor fibres of the Par Vagum, which, 

 being conveyed to the Medulla Oblongata, shall give rise to the transmission 

 of motor impulses to the several muscles, that shall combine them in the act 

 of coughing. This act consists, 1st, in a long inspiration, which fills the 

 lungs ; 2d,< in the closure of the glottis at the moment when expiration com- 

 mences; and 3d, in the bursting open (as it were) of the glottis, by the vio- 

 lence of>4he expiratory movement ; so that a sudden blast of air is forced up 

 the air-passages, carrying before it any thing that may offer an obstruction. 

 The difference between coughing and Sneezing consists in this, that in the 

 latter, the communication between the larynx and the mouth is partly or 

 entirely closed, by the drawing together of the sides of the velum palati over 

 the back of the tongue ; so that the blast of air is directed, more or less com- 

 pletely, through the nose, in such a way as to carry off any source of irrita- 

 tion that may be present there. 



190. The influence of the Spinal Cord, and of its system of nerves, on the 

 movements of Respiration, affords an excellent example of the importance of 

 this organ, as supplying the conditions immediately requisite for the mainte- 

 nance of the organic functions. We have seen that, strictly speaking, the 

 act of Respiration, as we commonly understand it, is not Respiration itself ; 

 for this consists in the interchange of ingredients between the blood and the 

 surrounding medium, which is effected in the air-cells of the lungs, and which 

 takes place in the lower animals (as in plants) without any muscular effort. 

 But, in proportion to the necessity for the energetic exercise of this function, 

 do we find a special provision in the higher classes, for the constant renewal 

 of that portion of the surrounding medium which is in contact with the aerat- 

 ing surface; and this comes to be so necessary, that asphyxia might be pro- 

 duced, without any interruption to the ingress of air through the trachea, by 



