THE HEAD AND NECK rioy 



inaccessible. The thyroid body is intimately related to it superiorly. 

 Each lateral lobe closely embraces it laterally as low as about the 

 fifth ring, and the isthmus lies in front of the second and third rings 

 as a rale, but its position is liable to variation. Crossing the upper 

 border of the isthmus there is one of the branches of the superior 

 thyroid artery, known as the artery of the isthmus. 



There is nothing of any importance in front of the trachea above 

 the isthmus of the thyroid body. Below the isthmus there is a more 

 or less copious plexus of veins, called the inferior thyroid plexus, from 

 which the right and left inferior thyroid veins descend. Occasionally 

 a small artery, called the arteria thyroidea ima, ascends directly in 

 front of this part of the trachea, in the median line, to reach the 

 isthmus of the thyroid body. The innominate, and even the right 

 common carotid, artery, and the left innominate vein sometimes 

 encroach upon the front of the trachea towards the root of the 

 neck. In early life the upper part of the thymus body covers the 

 front of the trachea. The foregoing structures are covered by the 

 stemo-thyroid and stemo-hyoid muscles in the following manner : 

 the two stemo-thyroid muscles are in contact with each other for 

 a short distance above the manubrium stemi, so as to cover the 

 trachea, but the two stemo-hyoid muscles are here separated by 

 an interval ; superiorly the two stemo-thyroid muscles diverge, 

 and the two stemo-hyoid muscles come very nearly together. 



The operation performed upon the trachea is tracheotomy. 

 According as it is performed above or below the isthmus of the 

 thyroid body, it is spoken of as the high operation, or the low 

 ope ation. In the high operation there is no anatomical obstacle, 

 unless it be a close attachment of the isthmus of the thyroid 

 body to the tracheal rings which it covers. In the low operation 

 the following obstacles are presented : (i) the trachea is here less 

 accessible, because it recedes from the surface ; (2) the inferior 

 thyroid plexus of veins might prove troublesome ; (3) an arteria 

 thyroidea ima may be present ; (4) the innominate and right common 

 carotid arteries, and the left innominate vein may be endangered ; 

 and (5) in young children the thymus body would be in the way. 



Descendens Cervicis (Descendens Hypoglossi) Nerve. — ^The de- 

 scendens cervicis nerve arises from the hypoglossal nerve as the 

 latter hooks round the occipital artery, its fibres being derived from 

 the communicating branch which the hypoglossal receives from 

 the loop between the first and second cervical nerves. The nerve, 

 which is long, passes downwards and slightly forwards, lying 

 upon, or within, the carotid sheath, and in either case directly 

 over the line of the common carotid artery. Before reaching the 

 centre of the neck it fumishes a branch to the anterior belly of the 

 omo-hyoid muscle. Thereafter it is joined by a branch which is 

 formed by the union of the two rami communicantes cervicis from 

 the anterior primary divisions of the second and third cervical 

 nerves. These two rami, however, sometimes join it separately. 

 In this manner a loop is formed, which is called the ansa cervicis 



