TOPOGRAPHY OF THE THORAX AND ABDOMEN 63 



The lateral lobes are connected to each other in their 

 lower halves by the isthmus, which lies upon the anterior 

 surface of the trachea over the second, third and fourth 

 tracheal rings. It is difificult to distinguish the lines of 

 separation between the lateral lobes and the isthmus 

 (Plate IV). From the upper border of the isthmus a small 

 pyramidal lobe rises. It lies over the first tracheal ring 

 and the arch of the cricoid cartilage. 



TABLE OF LEVELS 



In the following table of levels, the thoracic vertebrae 

 are denoted by the letter T, with a subscript to indicate 

 the particular vertebra in question. The lumbar vertebrae 

 are denoted by the letter L in the same way. The interver- 

 tebral discs are denoted by the letter D with the subscripts 

 1 to 12, for those in the thoracic region, and 1 to 5 for those 

 in the lumbar region. The number of the disc corresponds 

 to the number of the vertebra immediately above. The tips 

 of the spinous processes are denoted by the letter S. The 

 same subscripts are vised as in the case of the centra. R and 

 C are the abbreviations used for rib, and costal cartilage. 

 On account of the fact that the vertebral column is the 

 axis of the skeleton, and especially on account of its being 

 divided into alternate centra and discs which are convenient 

 as points of reference it has been taken as the basis of this 

 table. Only those points which are fixed and at the same 

 time readily accessible, have been used in establishing levels 

 upon the anterior body wall. The tips of the spinous pro- 

 cesses have been used not because they are fixed and con- 

 stant but because they are the only accessible points upon 

 the posterior body wall. 



The table is based upon imaginary horizontal sections 



