972 SPECIAL PHYSIOLOGY. 



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Thus, the lungs commence by two solid cellular processes, from 

 the front of the oesophagus, which soon become hollow. These 

 processes branch out into numerous, at first solid, but afterwards 

 hollow, ramifications, which become lined by a ciliated epithelium, 

 and form the bronchi, bronchia, and air-cells. The primitive lungs 

 open separately into the pharynx, but afterwards by a common tra- 

 chea. The trachea and larynx are produced by a lengthening out 

 and excavation of a primitive peduncle, derived from the oesophageal 

 walls: the larynx is developed from two lateral symmetrical masses 

 of blastema, connected in front. Until the lungs are inflated at birth, 

 they resemble a solid gland containing ducts with terminal acini ; 

 they then lie .close to the back of the thorax. 



The liver first appears, as two small conical projections of the co- 

 rium and epithelial tissue, on the side of the intestine, below the stom- 

 ach. These soon become hollow, and from them, numerous, solid, 

 cylindrical branches extend into the growing matrix. Continuing to 

 extend, these ramified masses of cells ultimately unite in a terminal 

 network, and, becoming hollowed out, form the bile-ducts. The liver 

 rapidly acquires a relatively large size, and is lobed ; it soon even 

 begins to secrete bile, or, at least, the coloring substance of the bile, for 

 the biliary acids are said to be absent. The imperfect hepatic secretion 

 enters the intestine of the embryo, forming the so-called meconium. 



The pancreas is developed, in a similar manner, close to the spleen, 

 by the formation of a small mass of cells from the epithelial layer on 

 the left side of the intestine, which afterwards grows, and becomes 

 canaliculated to form the pancreatic ducts. 



The spleen commences near the great curvature of the stomach, 

 but probably from a distinct blastodermic mass. The thyroid body 

 appears as a similar small mass, in contact with the oesophageal lining 

 membrane, in close connection with the commencing larynx. The 

 thymus is said to originate separately, in front of the great vessels of 

 the neck, as a delicate closed tube, which becomes diverticulated at 

 the sides, and filled with nucleated cells and fluid. 



The Urinary and Reproductive Organs. 



The bladder, as already stated, is a part of the urogenital sinus, 

 and is connected at its apex with the urachus, or obliterated abdomi- 

 nal portion of the allantois. It is developed originally, as a small 

 pouch, connected with the lower end of the large intestine. The part 

 of the intestine below the primitive communication with the bladder, 

 forms a true cloaca, into which the digestive, urinary, and reproduc- 

 tive canals all open. The anterior part is soon separated from the 

 proper intestinal portion by a septum which corresponds with the 

 future perineum. It is this anterior part which forms, for a time, 

 the sinus urogenitalis, the common outlet of the urinary and repro- 

 ductive passages, and which becomes ultimately modified according to 

 the sex. 



Very early in embryo life, two remarkable symmetrical organs ap- 

 pear, at first as two linear elevations, one on each side of the primitive 



