762 



THE THYROID BODY. 



the ninth day the whole is invested by a 

 capsule of connective tissue, which sends 

 in septa dividing it into a number of lobes 

 or solid masses of cells, and by the six- 

 teenth day it is a paired body composed of 

 a number of hollow branched follicles, each 

 with a ' membrana propria,' and separated 

 from each other by septa of connective 

 tissue. It finally travels back to the point 

 of origin of the carotids. 



Amongst Mammalia the thyroid arises 

 in the Rabbit (Kolliker) and Man (His) as 

 a hollow diverticulum of the throat at the 

 bifurcation of the foremost pair of aortic 

 arches. It soon however becomes solid, 

 and is eventually detached from the throat 

 and comes to lie on the ventral side of the 

 larynx or windpipe. The changes it under- 

 goes are in the main similar to those in the 

 lower Vertebrata. It becomes partially 

 constricted into two lobes, which remain 

 however united by an isthmus 1 . The fact 

 that the thyroid sometimes arises in the 

 region of the first and sometimes in that of 

 the second cleft is probably to be explained 



Tli 



FIG. 417. SECTION THROUGH 

 THE HEAD OF AN ELASMOBRANCH 

 EMBRYO, AT THE LEVEL OF THE 

 AUDITORY INVOLUTION. 



Th. rudiment of thyroid body ; 

 aup. auditory pit ; aim. ganglion 

 of auditory nerve ; iv. v. roof of 

 fourth ventricle ; a.c.v. anterior 

 cardinal vein ; aa. aorta ; f.aa 

 aortic trunk of mandibular arch ; 

 //. head cavity of mandibular 

 arch ; Ivc. alimentary pouch which 

 will form the first visceral cleft. 



by its rudimentary character. 



The Thymus gland. The thymus gland may conveniently be 

 dealt with here, although its origin is nearly as obscure as its function. It 

 has usually been held to be connected with the lymphatic system. Kolliker 

 was the first to shew that this view was probably erroneous, and he 

 attempted to prove that it was derived in the Rabbit from the walls of one 

 of the visceral clefts, mainly on the ground of its presenting in the embryo 

 an epithelial character. 



1 Wolfler (No. 571) states that in the Pig and Calf the thyroid body is formed as a 

 pair of epithelial vesicles, which are developed as outgrowths of the walls of the first 

 pair of visceral clefts. He attempts to explain the contradictory observations of other 

 embryologists by supposing that they have mistaken the ventral ends of visceral 

 pouches for an unpaired outgrowth of the throat. Stieda (No. 569) also states that in 

 the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair 

 of visceral clefts, at a much later period than would appear from the observations of 

 His and Kolliker. In view of the comparative development of this organ it is 

 difficult to accept either Wolfler's or Stieda's account. Wolfler's attempt to explain 

 the supposed errors of his predecessors is certainly not capable of being applied in 

 the case of Elasmobranch Fishes, or of Petromyzon ; and I am inclined to think that 

 the method of investigation by transverse sections, which has been usually employed, 

 is less liable to error than that by longitudinal sections which he has adopted. 



