chap, ix.] THE LARYNX AND TRACHEA. 135 



The tubes below 12 mm. are for children, and are 

 divided into four sets. 



No. 2. 8mm. 4 to 8 



No. 3. 10mm. 8 to 12 



No. 4. 12mm. 12 to 15 



In children under eighteen months the diameter of 

 the tube should be about 4 mm. 



In performing tracheotomy it is most impor- 

 tant that the head be thrown as far back as possible, 

 and that the chest be kept strictly in a line with 

 the sternal notch, so that the relations of the middle 

 line of the neck be preserved. Full extension of the 

 head not only gives the surgeon increased room for 

 the operation, but also brings the trachea nearer to 

 the surface, and by stretching the tube renders it 

 much less mobile. 



In cutting down upon the trachea in the middle 

 line of the neck from the cricoid cartilage to the 

 'sternum the following parts are met with. Beneath 

 the integument lie the anterior jugular veins. As 

 a rule these veins lie some little way apart on either 

 side of the median line, and do not communicate 

 except by a large transverse branch which lies in the 

 iiiterfascial space at the upper border of the sternum. 

 Sometimes there are many communicating branches 

 right in front of the tracheotomy district, or the veins 

 may form almost a plexus in front of the trachea, or 

 there may be a single vein which will follow the 

 middle line. Then comes the cervical fascia, enclosing 

 the sterno-hyoid and sterno-thyroid muscles. The 

 gap between the muscles of opposite sides is lozenge- 

 shaped, and is such that the trachea can be exposed 

 without dividing muscle fibres. The isthmus of the 

 thyroid usually crosses the 2nd, 3rd, and 4th rings of 

 the trachea. Above it a transverse communicating 



