no 



THE THYKOID BODY. 



Fig. 133. LUNGS OF A. HUMAN EMBRYO 



STILL MORE ADVANCED. (His.) 



progresses in growth, giving off as it proceeds lateral diverticula, which form the secondary 

 bronchi, and these again giving- off others until the whole complicated bronchial ramification 

 is eventually produced. Like the first sprouts from the median diverticulum. all the secondary 

 and other sprouts are dilated at their termination, and have a lobulated aspect (fig. 125. Ly ; 



figs. 181. 132, 133. This is due to the fact that they 

 are undergoing a further division or sprouting. This 

 process goes on until the sixth month of intrauterine 

 life, by which time all the dilated ends of the growing 

 and sprouting tubes have reached the surface of the 

 lung. These dilated extremities which now appear 

 grouped together, and apparently springing several 

 from a common tubs, form the infundibula, but their 

 walls are not at first beset with air-cells. The forma- 

 tion of these takes place when the bronchial ramifica- 

 tion is completed (sixth month. Kolliker). as small, 

 closely-set, pouch-like protrusions of the walls of the 

 infundibula. and of the terminal bronchial tubes. 



The trachea and larynx are formed by a 

 separation from the oesophagus of the original 

 median diverticulum, from the lower angles of 

 which the bronchial rudiments have sprung, the 

 separation commencing below, and leaving a 

 relatively small connection between the two 



tubes above : this connection is the rudimentary glottis. As development advances, 

 both the tracheo-laryngeal and the oesophageal tubes lengthen, the latter relatively 

 more than the former, so that the lung rudiments no longer lie, as was the case at 

 first, in front of and on either side of the stomach, but extend downwards somewhat 

 short of that organ (fig. 125), separated from one another by the oesophagus behind, 

 and the heart and pericardium in front. As they thus grow backwards with the 

 lengthening of the trachea, the lung rudiments project into the anterior part of the 

 body-cavity or coelom (dorsal portion), and receive a covering from its lining mem- 

 brane, at first only below and on the external surface, but subsequently on the 

 internal aspect, so as to separate them from the oesophagus. The portions of the 

 body-cavity into which the lungs project become shut off from the remainder on 

 the formation of the diaphragm and pericardium, and form the pleurae. 



The pulmonary blood-vessels are comparatively late in being developed, the 

 arteries penetrating into the lung tissue only on the twelfth day in the chick. 



The thyroid body is developed partly as a median diverticulum of the pharyn- 

 geal hypoblast opposite the ventral ends of the second visceral arches (fig. 125, B, Sd); 

 partly as a (bilateral) diverticulum of the posterior wall of the fourth visceral cleft. 

 The median diverticulum in most animals early becomes separated from the 

 pharyngeal hypoblast, and is thus converted into an island of epithelium imbedded 

 in mesoblast. In the human embryo, as His has shown (fig. 134, A, thr\ it remains 

 for some time in the form of a hollow bifid vesicle, which is connected with the upper 

 surface of the tongue by a small duct (ductus thyrcoglossus, d) ; subsequently, however, 

 the vesicle becomes solid, and the duct is obliterated and disappears, with the 

 exception of a small portion near the orifice, which becomes converted into the 

 foramen ccvcum of Morgagni,/.^. 



Occasionally even in the adult a comparatively long duct is found, leading downwards 

 and backwards from the foramen caecum. This, which has been termed the ductus Ihtf/itdfis. 

 is the remains of the original thyrolingual duct connecting the median part of the thyroid 

 with the tongue. It may further happen that the lower part of this connection also remains 

 in the shape of a tubular prolongation of the median portion of the thyroid towards the root 

 of the tongue (r/?/^?/.<? thryoldnis ; when well developed this forms the -pyramid). The so- 

 called accessory thyroid bodies (supra-hyoid. prsehyoid glands, &c.) which are occasionally 

 found near the hyoid bone, are also referable to the thyrolingual duct (His. Anatomie 



