26 OUTLINES OF ANATOMY. 



ascend. A narrow, lozenge-shaped space is thus left between the inner borders of 

 these muscles. Over this area the trachea is not covered by any muscular structure. 



SURGICAL ANATOMY. (From Cunningham.) 



The principal operations which are performed in the middle line of the neck are those of laryng- 

 otomy and tracheotomy. 



In laryngoiomy, an opening is made into the larynx. This can most readily be done in the inter- 

 val between the thyroid and cricoid cartilages. A vertical mesial incision through the integument is 

 made over this interval. The crico-thyroid membrane is thus exposed, and is divided transversely close 

 to the upper margin of the cricoid cartilage. It is a very simple proceeding, and one which is attended 

 with little or no danger if ordinary care be taken. The crico-lhyroid membrane is divided transversely, 

 and in its lower part, for two reasons, viz., (i) to avoid injury to the crico-thyroid artery, which, 

 although, as a general rule, of small size and of no surgical importance, is sometimes large enough to 

 give rise to awkward results if wounded ; and (2) to place the opening as low down as possible. 



Tracheotomy is a more serious operation. The opening into the trachea may be made above 

 or below the isthmus of the thyroid body. The high operation is very properly preferred by the sur- 

 geon. Its advantages are very apparent : here the trachea lies near the surface, and no veins of any 

 importance are met with. The only drawback consists in the small portion of trachea which intervenes 

 between the isthmus and the cricoid cartilage. Still, this can be increased by pushing down the 

 isthmus, which, within certain limits, can be easily dislodged in a downward direction. Many surgeons, 

 indeed, consider that the wounding of the isthmus is a matter of comparatively slight importance. The 

 fact, however, that a large branch of the superior thyroid artery is generally found in relation to its 

 upper border should make the operator hesitate before having recourse, in all cases, to this expedient 

 for gaining additional space. In the child it is frequently necessary to combine the high operation of 

 tracheotomy with that of laryngotomy- -viz., by cutting through the cricoid cartilage. 



The low operation is a formidable undertaking. It is true that there is a greater length of 

 tube to be operated upon ; but this is situated very deeply, and the surgeon encounters many difficulties 

 before it is reached. If the dissector reflect upon the structures which intervene between this part of 

 the trachea and the surface, he will fully realize this; and he must bear in mind that these difficulties 

 are greatly intensified in the living subject by the engorged state of the veins and the convulsive move- 

 ments of the windpipe as the patient struggles for breath. In the child, the thymus body interposes an 

 additional obstacle; and this, combined with the more limited space, the small calibre and great 

 mobility of the trachea, render the operation, in such cases, a very serious responsibility. In the low 

 operation, the trachea must be opened in an upward direction, so as to avoid injury to the innominate 

 artery and left innominate vein, which are placed in front of it at the upper margin of the sternum. 



Dissection. 



" Dissection. The numerous and diverse structures contained within the anterior triangle must now be 

 displayed. The dissection should be carried out over the entire area at once, and the structures found 

 in one subdivision followed upward or downward, as the case may be, into the other subdivisions of the 

 space. It is a common fault with dissectors to fail to open up the digastric triangle until the two lower 

 triangles have been fully dissected. Two small nerves are especially liable to injury, and therefore 

 should be secured as early as possible. They are the thyro-hyoid branch of the hypoglossal nerve, and 

 the external laryngeal nerve. The hypoglossal nerve, which will be found crossing the carotid triangle 

 at the lower border of the posterior belly of the digastric, should be traced forward ; as it approaches 

 the hyoid bone, its minute thyro-hyoid branch will be discovered, leaving its lower border at an acute 

 angle, and proceeding downward and forward to reach the thyro-hyoid muscle. The external laryn- 

 geal nerve is a long, slender branch which occupies a deeper plane. To expose it the carotid vessels 

 should be pulled outward from the larynx, and the loose tissue in the interval thus opened up divided 

 carefully in an oblique direction and along a line connecting the cricoid cartilage with the bifurcation 

 of the common carotid artery. The nerve will be found as it passes downward and forward to disappear 

 under cover of the depressor muscles of the larynx. It will be traced to its ultimate distribution at a 

 subsequent stage of the dissection." (Cunningham.) 



Hypoglossal nerve, 820-1. (801-2) 



Descendens hypoglossi : 



Ansa hypoglossi. 

 Sterno-hyoid, 475-6. (468-9) 



Origin. Insertion. Structure. Nerve-supply. Action. Relations. Variations. 



Divide the sterno-hyoid transversely about an inch above the sternum, raise the upper portion as far as 

 the insertion, reflect the lower portion to the origin. Expose the sterno-thyroid and thyro-hyoid 

 muscles, then replace the sterno-hyoid. 



Sterno-thyroid, 477. (470) 



Origin. Insertion. Structure. Nerve-supply. Action. Relations. Variations. 

 Thyro-hyoid, 477. (470-1) 



Origin. Insertion. Structure. Nerve-supply. Action. Relations. Variations. 



