ANATOMY IX A NUTSHELL. 



247 



of an inch in thickness, and an inch and a quarter in breadth. It extends from 

 the middle of the thyroid cartilage to the sixth ring of the trachea, and the 

 isthmus crosses the trachea about the third ring. The isthmus may. in rare 

 cases, pass behind the trachea in front of the oesophagus. This gland weighs 

 from one to two ounces. It is larger in the female and is increased in size dur- 

 ing menstruation. 



Blood Supply. — The superior thyroid arteries, which are branches of the 

 external carotid, supply the apex and inner and fore parts of the lateral lobes. 

 The inferior thyroid arteries, which come from the thyroid axis, supply the 



PLATE C. 



A Lumbar Vertebra. 



outer and posterior portion of the lateral lobes. The thyroidea ima. which is 

 derived either from the innominate artery or from the arch of the anna, ascends 

 on the front of the trachea to anastomose with the superior and inferior thy- 

 roid arteries and helps to supply the isthmus. The veins arc the superior 

 thyroid which empties into the internal jugular, and the middle thyroid which 

 empties into the same, and the inferior thyroid which empties into the innom- 

 inate of the corresponding side. 



Nerve Supply. — From the superior and middle cervical ganglion of the 

 sympathetic, and possibly a branch from the tenth. 



A goiter may be distinguished from a cervical tumor by the fact thai it 

 rises ami falls on deglutition. 



