558 



A MANUAL OF PHYSIOLOGY 



disappear in the adult. Islands of thymus tissue are found 

 at all ages among the fat by which the bulk of the organ is 

 replaced. The chief effect of intravenous injection of extract 

 of human or ox thymus is a lowering of blood-pressure. In 

 this respect it resembles thyroid extract. The heart may be at 

 the same time accelerated. 



Thyroids and Parathyroids. The thyroid consists of two 

 lobes connected by an isthmus across the middle line in man 

 and some animals, but often separate. In the neighbourhood 



of the thyroid, or em- 

 bedded in its tissue, are 

 certain bodies called para- 

 thyroids, consisting of 

 solid columns of epithelial 

 cells. The number and 

 6 situation of the parathy- 



V ?-^ f! %v.ffel*^ roids are n t constant. 



As a rule, there are four 

 in mammals, two on each 

 side, but this number is 

 subject to variations in 

 different individuals of 

 the same species. The 

 variability in their ana- 

 tomical relations to the 

 FIG. 187. PARATHYROID (VINCENT AND thyroid is of greater signi- 

 ficance. For much of the 



A small portion of parathyroid of cat , , -, -, 



embedded in thyroid tissue. It consists for Uncertainty in which the 



the most part of solid columns of epithelial whole question of the 



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



nective tissue (6). A thyroid vesicle (n) and , V . ,, ,, 



portions of two others (i, 10) are seen in the tir patlOn OI the thyroids 



lower part of the figure, separated from the was Until lately involved 



parathyroid by a fibrous capsule (2) 4 , 7, arose from ignorance or 



bloodvessels ; 9, lower boundary of the para- . -~ . ... . 



thyroid tissue. ( x 500.) insufficient recognition of 



this variability. In most 



animals the inferior, anterior, or external pair of parathy- 

 roids is more or less distinctly separated from the thyroid. 

 The separation is especially evident in the herbivora, in the 

 monkey, and in man, and this pair of parathyroids 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 herbi- 

 vora and carnivora, is always very closely associated 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 



TV o c r^kiV^a 



