150 PHYSIOLOGY. 



vertebrae ; it runs down the neck, and enters the chest on 

 the outer side of the internal jugular vein. 



17. The external respiratory nerve is distributed to the 

 intercostal muscles, and those which extend from the ribs to 

 the shoulder, and which are occasionally employed in labo- 

 rious breathing. It is through the instrumentality of the 

 accessory, phrenic, and external respiratory, that the mus- 

 cles employed in respiration are brought into action, com- 

 bined and directed with the proper degree of force, velocity, 

 and extent, without the necessity of interference of the 

 mind. Though to a certain extent, they may be under the 

 influence of the will, yet it is only in a secondary degree. 

 No one, for example, can long suspend the movements of 

 respiration, for in a short time, instinctive feeling issues its 

 irresistible mandates, which neither requires the aid of erring 

 reason, nor brooks the capricious interference of the will.) 



18. The influence of this order of nerves in the expression 

 of the passions, is strikingly depicted in Sir Charles Bell's 

 Treatise on the Nervous System. " In terror," he remarks, 

 " we can readily conceive why a man stands with his eyes 

 intently fixed on the object of his fears, the eye-brows eleva- 

 ted, and the eye-balls largely uncovered ; or, why with hesi. 

 tating and bewildered steps, his eyes are rapidly and wildly 

 in search of something. In this way we only perceive the 

 intense application of his mind to the objects of his appre- 

 hension, and its direct influence on the outward organs. 

 But when we observe him farther, there is a spasm in his 

 breast ; he cannot breathe freely ; the chest remains elevated, 

 and his respiration is short and rapid ; there is a gasping 

 and convulsive motion of his lips, a tremor on his hollow 

 cheeks, a gasping and catching of his throat ; his heart 

 knocks at his ribs, while yet there is no force in the circu- 

 lation, the lips and cheeks being ashy pale." 



19. " To those I address, it is unnecessary to go farther 

 than to indicate that the nerves treated of in these papers 

 are the instruments of expression, from the smile upon the 





