THE NERVOUS SYSTEM 52 1 



Chauveau and others, and recently Sisson, described it as being 

 innervated by the external laryngeal nerve. The function of 

 the crico-thyroid muscle being to render the vocal cords tense, 

 division of the nerve produces a hoarse voice. Section of the 

 superior laryngeal causes loss of sensibility in the larynx, and 

 allows food to enter it. The inferior or recurrent laryngeal nerves 

 are given off from the vagus within the chest. The right is the 

 first, being given off opposite the second rib, and winding around 

 the dorso-cervical artery from without inwards, while the left 

 is given off above the base of the heart, the nerve winding from 

 without inwards around the posterior aorta. Both nerves leave 

 the thoracic cavity and return up the neck. This complicated 

 arrangement resembles very closely the ' out-of-the-way ' course 

 taken by the spinal accessory in order to reach the trapezius. 

 From the point where each recurrent is given off within the chest 

 their course is not identical ; the left, for instance, having a 

 greater distance to travel, is somewhat mixed up with the struc- 

 tures around the base of the heart, while in its passage up the neck 

 it is more superficially placed than the right recurrent. Both 

 nerves supply motor power to all the muscles of the larynx, 

 with the exception of the crico-thyroid. All the fibres in the 

 recurrents are not motor ; sensory branches are given off to the 

 trachea and oesophagus in their passage up the neck. At p. 148 

 sufficient consideration has been devoted to the subject of 

 paralysis of the laryngeal muscles and its influence on respiration. 

 The question is of intense practical interest, owing to the frequency 

 with which the innervation of these muscles is destroyed on the 

 left side. At present there is no satisfactory explanation of this 

 unilateral paralysis. After division of both recurrent nerves, 

 both sides of the larynx are paralysed, and in horses asphyxia 

 is gradually produced. The writer has, however, seen bilateral 

 paralysis not seriously interfering with slow work. In such cases 

 it is believed that the age of the horse is the saving factor ; the 

 cartilages, becoming rigid with age, prevent the arytenoids from 

 completely collapsing over the opening of the glottis. If the 

 recurrent nerve on either side of the neck be divided and the horse 

 galloped after the operation, he is found to be a ' roarer.' Not 

 only does paralysis of these muscles interfere with respiration, 

 but it also affects the voice. The altered character of the voice 

 of the horse may be observed either during neighing or coughing. 

 So distinctive, indeed, is the cough of a ' roarer,' that such cases 

 may be at once recognised. Longet found that dividing the 

 recurrent in old animals led to loss of voice, while in the young 

 the voice was rendered unnatural and shrill. This shrill con- 

 dition was entirely lost by cutting the external laryngeal, which, 

 by paralysing the cricothyroid, prevented the vocal cord from 



