THYREOID GLAND 229 



the oesophagus and the pharynx, and the recurrent nerve. 

 In most cases it overlaps the common carotid artery. Its 

 anterior border is connected with the anterior border of the 

 opposite lobe by the isthmus. Above the isthmus it is in 

 relation with the anterior terminal branch of the superior 

 thyreoid artery, and, below the isthmus, with the commence- 

 ment of the inferior thyreoid vein. 



The isthmus of the thyreoid gland has already been seen 

 in the dissection of the middle line of the neck. It is a 

 band of varying width which Ires anterior to the second, third, 

 and fourth rings of the trachea, and, therefore, nearer the 

 lower than the upper ends of the two lobes. 



An additional lobe, the pyramidal or middle lobe, is 

 frequently present. It is an elongated slender process which 

 springs from the isthmus, on one or other side of the median 

 plane (more usually on the left side), and extends upwards 

 towards the hyoid bone. It may be connected to the hyoid 

 bone by fibrous tissue, or by a narrow slip of muscular fibres 

 called the levator glandulcz thyreoidece. That little muscle, in 

 some cases, has an attachment to the thyreoid gland independ- 

 ently of the pyramidal process. The thyreoid gland is firmly 

 connected by fascia to the parts upon which it lies, and 

 therefore follows the larynx in all its movements. 



The dissector should note the great vascularity of the 

 thyreoid gland. Four large arteries, and occasionally a fifth 

 smaller vessel, convey blood to its substance. At the apex 

 of each lobe a superior thyreoid branch of the external 

 carotid artery divides into three branches which supply the 

 gland ; the two inferior thyreoid branches, from the thyreo- 

 cervical trunks of the subclavian arteries, distribute their 

 terminal branches to the basal portions and deep surfaces of 

 the two lobes. The occasional artery is the thyreoidea ima, 

 a branch of the innominate or, more rarely, of the common 

 carotid or the aortic arch. It ascends, upon the anterior 

 aspect of the trachea, to reach the isthmus of the thyreoid 

 gland. These thyreoid arteries anastomose with one 

 another. 



The veins which drain the blood away from the thyreoid 

 gland are still more numerous. They arise, in part, by 

 tributaries which spring from a venous network on the 

 anterior surface of the gland, but chiefly by branches which 

 emerge from its substance. They are three in number on 

 in 156 



