THE NECK 161 



genio-glossi, and if it begins sufficiently far back it emerges 

 through the dorsum of the base of the tongue. The lingual 

 vessels and hypoglossal nerve are divided, but the lingual nerve 

 usually escapes as it lies above the level of the wound. More 

 posteriorly, the external maxillary (facial) artery and common 

 facial vein which is usually the first large vessel to be severed 

 are reached, but the carotid sheath is only injured when the 

 incision is a very large one. 



The infra-hyoid wound closely resembles the incision for 

 sub-hyoid pharyngotomy (p. 159). Laterally, it cuts through 

 the superior thyreoid artery and internal laryngeal nerve before 

 reaching the sterno-mastoid and the carotid sheath. The 

 epiglottis is apt to be divided, and when it is cut its upper part 

 may cause respiratory obstruction. 



" Cut throat " wounds through or below the thyreoid car- 

 tilage are by no means common. 



The crico-thyreoid muscle arises from the side of the cricoid 

 cartilage, and passes upwards, backwards, and laterally, to be 

 attached to the thyreoid cartilage. When the crico-thyreoid 

 contracts it elevates the anterior part of the cricoid cartilage, 

 as the thyreoid is held firmly in place by the sterno-thyreoid and 

 thyreo-hyoid muscles. This movement depresses the posterior 

 part of the cricoid together with the arytenoids, and thus 

 separates the posterior from the anterior attachment of the vocal 

 folds, rendering them tense. The external laryngeal nerve, 

 which supplies the muscle, is a branch of the superior laryngeal 

 (p. 159), and lies in close relation to the upper pole of the lateral 

 lobe of the thyreoid gland. Should the nerve be stimulated by 

 the pressure of forceps during operations in its neighbourhood, 

 the resulting muscular response may be so strong as to cause 

 spasm of the glottis. 



The condition of laryngismus stridulus is due to reflex stimu- 

 lation of the external laryngeal nerve. It may be caused by 

 direct pressure on the main trunk of the vagus itself from 

 tuberculous abscesses or enlarged lymph glands in the thorax. 

 Spasm of the glottis may also occur in gastric disorders, in which 

 case the afferent impulse travels along the vagus itself, and in 

 the disturbance due to dentition, when the afferent impulse is 

 carried up to the sensory nucleus of the trigeminal, from which 

 it spreads to the motor nucleus of the vagus. 



As the two crico-thyreoid muscles diverge from each other 

 the crico-thyreoid ligament (middle part of crico-thyreoid 



11 



