THYROIDS AND PARATHYROIDS 



631 



or ox thymus is a lowering of blood-pressure; but there is nothing 

 specific in this, a similar effect being given by thyroid extract and 

 the extracts of majiy other tissues. The heart may be at the same 

 time accelerated.! ^ 



Thyroids and /irathyroids.-f-The thyroid consists of two lobes 

 connected by an isthmus across the middle line in man and some 

 animals, but often separate. Iftn the neighbourhood of the thyroid, 

 or embedded in its tissue, are certain bodies called parathyroids! 

 consisting of solid columns of epithelial cells. Ihe mlfhber and 

 situation of the parathyroids are not constant. As a rule, there are 

 four in mammals, two on 

 each sicTey but this number 

 is subjecr to variations in 

 different individuals of the 

 same species. /The varia- 

 bility in their * anatomical 

 relations to the thyroid is 

 of greater significance^ For 

 much of the uncertainty in 

 which the whole question of 

 the symptoms following ex- 

 tirpation of the thyroids was 

 until lately involved arose 

 from ignorance or insufficient 

 recognition of this variability. 

 /In most animals the inferior, 

 anterior, or external pair of 

 parathyroids is more or less 

 distinctly separated from the 

 thyroid.! The separation is 

 especialry evident in the her- 

 bivora, in the monkey, and 

 in man, and this pair of para- 

 thyroids is much larger than 

 the other. In carnivorous 

 animals, as the dog and cat,the anterior pair of parathyroids is closely 

 adherent to the thyroid capsule. The superior, posterior, or internal 

 pair, both in herbivora and carnivora, is always very closely associ- 

 ated with the capsule of the thyroid, and frequently embedded in the 

 substance of the gland. The consequence of this arrangement is 

 that in the older experiments the chief masses of parathyroid tissue 

 were much more likely to escape removal with the thyroid in the 

 case of herbivorous than in the case of carnivorous animals. 

 /" But even in one and the same species considerable variations may 

 /exist. It is easy to see, then, that in removing the thyroid the 

 I parathyroids would sometimes be completely removed as well, 



Fig. 202. Parathyroid (Vincent and Jolly). 

 A small portion of parathyroid of cat em- 

 bedded in thyroid tissue. It consists for 

 the most part of solid columns of epithelial 

 cells (3, 5, 8) with strands of vascular con- 

 nective tissue (6). A thyroid vesicle (n) 

 and portions of two others (r, 10) are seen 

 in the lower part of the figure, separated 

 from the parathyroid by a fibrous capsule 

 (2). 4, 7, bloodvessels; 9, lower boundary 

 of the parathyroid tissue. ( x 500.) 



