THE PARATHYEEOID GLANDS. 



1349 



Internal jugular vein 

 Vagus nerve 



01 to 3 gm. in weight. Most commonly they are lenticular, 5-7 mm. in length, 

 1-2 mm. in thickness, and from -01 to -1 gm. in weight. Normally there are two pairs 

 of parathyreoids, distinguished by the Roman numerals IV. and III. to signify that 

 they develop from the fourth and third pharyngeal pouches. Sometimes, in course 

 of development, the parathyreoid buds divide so that more than four, five to twelve, 

 glands may be present: the numerals are then applied to the groups of glands 

 formed from the pouches. 



Parathyreoid IV. is commonly embedded in the tunica propria of the thyreoid gland 

 'and lies posterior to the corresponding lateral lobe of that organ, about its middle. Para- 

 thyreoid III., similarly embedded, usually lies on the posterioi aspect of the inferior ex- 

 tremity of the lateral lobe of the thyreoid gland. As a rule the anastomosing channel, 

 which connects the inferior and superior thyreoid arteries (see Thyreoid Gland, Blood 

 'Supply), passes near both parathyreoids and furnishes the best guide to their discovery, but 

 the range of the exceptional positions which the glands may occupy is wide. Thus, para- 

 thyreoid IV. may be found (1) behind the pharynx or oesophagus, (2) in the fibrous tissue 

 at the side of the larynx, above the level of the thyreoid gland, (3) behind any part of the 

 corresponding lobe of the thyreoid gland or even embedded in the thyreoid substance 

 '(internal parathyreoid) ; whereas parathyreoid III. may be found (1) near the bifurcation 

 iof the common carotid artery, (2) behind any part of the corresponding lobe of the 

 thyreoid gland, (3) on the 

 sides of the trachea, or (4) 

 in the thorax. 



Blood-vessels. The 

 blood supply of each para- 

 thyreoid is effected by a 

 single artery which enters 

 the gland at its hilum. It 

 ' may spring from any branch 

 of the inferior or superior 

 thyreoid arteries, but most 



commonly is a branch of the Common carotid artery 

 large anastomosing channel innominate artery 



already referred to. 



Structure. The para- Subclavian artery 



thyreoids are built up of in- 

 tercommunicating trabecilla? Subclavian vein 

 af epithelial cells with 

 .strands of vascular connec- 

 tive tissue between them. 

 L?The cells are of two kinds, 

 Dne clear, the other, the min- 

 ority, containing oxyphyl 

 granules. Sometimes they 

 |r surround spaces recalling 

 fchyreoid vesicles ,but there 

 is no formation of " colloid," 

 except possibly after thy- 

 "eoidectomy. 



Development. The 

 parathyreoid bodies develop 

 from the dorsal diverticula 

 Jpf the third and fourth 

 ! i pharyngeal pouches. The 

 p first indication of their de- 

 velopment is a proliferation 

 | md thickening of the epi- 

 :helium on the cranial and 

 ateral aspects of the diver- 

 i icula. This is present in 

 )oth the third and fourth 

 xraches in 9-10 mm. embryos 

 )ut appears to be rather 

 rregular in the time of its 



ippearance. The cells forming it are vacuolated, difficult to stain, and indistinct in outline. 

 ; 3ords of cells grow out from the thickening and fibrous tissue penetrates between the out- 

 growing cords, which, soon lose their connexion with the pharynx. The differentiation of the 

 1 ;wo kinds of cells takes place after birth. 



Left lobe of thyreoid 

 gland 



Isthmus of thyreoid 

 gland 



Common carotid artery 

 ^ Internal jugular vein 



Band connecting 

 -" thymus with thyreoid 

 ^ Vagus nerve 



Subclavian artery 



Subclavian vein 

 Innominate vein 



Innominate 

 vein 



FIG 1059. THYMUS IN A FULL-TIME FCETUS^ HARDENED BY 

 FORMALIN-INJECTION. 



