266 



THE DUCTLESS GLANDS 



placed at the side of and towards the dorsal aspect of the 

 trachea, immediately aboral from the larynx. 



The parathyroids are usually four in number, but there 

 may be as many as five, and sometimes it is not possible to 

 see more than three with the unaided eye. The external 

 parathyroids are as a rule very easily found. Sometimes they 

 are at some distance from the thyroid 

 lobe, while at other times they are more 

 or less embedded in its substance, though 

 never so deeply as the internal glandule. 

 Sometimes their blood supply is such as 

 / 1 '-. 2 7T\ to permit of their being left behind in 



/ j * / \ the operation of thyroidectomy. This, 

 i however, is rare. The internal para- 

 thyroid, on the contrary, is small, deep 

 in the thyroid substance, and very vari- 

 able in position, so that in the living 

 animal the experimenter has frequently 

 to abandon the attempt to find it, while 

 even post mortem it cannot always be 

 revealed except by serial sections. It is 

 very rarely found that the internal 

 parathyroids can be seen on both sides 

 (see Figs. 60-65). 



(e) Cat. In the cat, as is most usual 

 throughout mammals, there are four 

 parathyroids, two internal and two 

 external. Their arrangement resembles 

 that in the dog. (Fig. 66.) 



(/) Rabbit. There are usually four parathyroids, but the 

 external glandules are uniformly placed at a considerable 

 distance from the thyroid lobe. For this reason, in tlio 

 earlier extirpation experiments they were always left behind 

 at the time of the operation. (See Figs. 67-71.) 



(g) In the Guinea-pig the distinction between internal and 

 external parathyroids seems largely to break down. There 

 are usually four parathyroids, sometimes three, or even only 

 two. (Figs. 72-76.) 



(h) In the Rat the thyroid consists of two lobes united by 

 an isthmus. There is only one parathyroid in connection 

 with each thyroid lobe, and this seems to correspond with 



FIG. 66. Shows the 

 relative positions of 

 the parathyroids, 

 thyroid, and trachea 

 in the cat. This 

 may be described as 

 an average condi- 

 tion of affairs. The 

 " internal " parathy- 

 roids are in dotted 

 outline. 



