THE THYROID BODY. 



197 



membranes. The visceral pleura consists of a stroma-layer, composed of 

 fibrous tissue intermingled with an abundance of elastic fibres, and a single 

 surface layer of mesothelial plates. The existence of definite openings or 

 stomata between these cells is doubtful. 

 The parietal pleura possesses a like struct- 

 ure, but is less rich in elastic fibres. The 

 subserous layer is scanty over the lung, 

 where it is continuous with the interlobular 

 connective tissue; over the mediastinum it 

 is firm and dense and on the costal wall 

 acquires the character of a fascia, which is 

 particularly dense beneath the apical pleura. 

 The blood-vessels supplying the vis- 

 ceral pleura are derivations of the pulmonary 

 trunks; those of the parietal pleura are 

 from various adjacent systemic branches. 

 In neither case is the ultimate distribution 

 a generous one, the twigs being small and 

 the capillaries comparatively few. The 

 lymphatics are most abundant over the 

 lungs and the intercostal spaces, where they 

 form meshworks within the stroma and 

 the subserous tissue. The nerves of the 

 visceral pleura include fibres from the vagi 

 and sympathetics by way of the pulmonary 

 plexuses. Those of the parietal pleura re- 

 ceive fibres from the intercostal and phrenic 

 nerves and, additionally, some from the vagi 

 and the sympathetics. Many of the sensory fibres are connected with 

 special end-organs, as the Pacinian and Golgi-Mazzonian corpuscles, while 

 others terminate in free varicose endings. 



iEndothelium 

 or free surface 



nnective-tissue 

 stroma of pleura 



FIG. 245. Section through free edge of 

 lung, showing visceral pleura. X 150. 



Solely as a matter of convenience, in view of their contiguity, the 

 Thyroid, Parathyroid and Thymus Bodies may be described in connection 

 with the respiratory tract. It must be clearly understood, however, that 

 these organs have neither morphological nor functional relations with the 

 organs of respiration ; they are, probably, to be regarded as accessory organs 

 of nutrition. 



THE THYROID BODY. 



The thyroid body is developed from an unpaired median rudiment. This 

 median rudiment or anlage is an eritodermic epithelial outgrowth from the 

 anterior wall of the primitive pharynx, in the region of the second visceral 

 arch and in close relation with the posterior part of the tongue. The position 

 of this outgrowth is later indicated by a depression on the tongue, the fora- 

 men caecum, just behind the apex of the row of circumvallate papillae. The 

 lateral outgrowths from the ventral wall of the fourth pharyngeal furrow do 

 not take part in forming the thyroid, as formerly supposed, but usually dis- 

 appear. The true anlage grows ventrally and subsequently forms the definite 

 thyroid surrounding the respiratory tube. The histogenesis of the organ 

 includes : (a) numerous cylindrical epithelial cords from which grow 

 out lateral branches ; (b~) fusion of these cords into a network whose 



