224 ANATOMY FOR NURSES [Chap. XII 



on in the proper direction. The numerous valves prevent a 

 return flow in the wrong direction. 



(3) Respiration. — During each inspiration the pressure on the 

 thoracic (hict is less than on the lymphatics outside the thorax, 

 and the lymph is accordingly " sucked " into the duct. During 

 the succeeding expiration the pressure on the thoracic duct is in- 

 creased, and some of its contents, prevented by the valve from 

 escaping below, are pressed out into the innominate veins. 



OEdema. — The hTiiph in the various IjTnph spaces of the body 

 varies in amount from time to time, but under normal circum- 

 stances never exceeds certain limits. Under abnormal condi- 

 tions, these limits may be exceeded, and the result is known as 

 oedema, or dropsy. Similar excessive accumulations may also 

 occur in the larger lymph spaces, the serous cavities. 



Among the possible causes of oedema are : — 



(1) An obstruction to the flow of lymph from the Ijinph spaces. 



(2) An excessive transudation, the lymph gathering in the 

 hmph spaces faster than it can be carried away by a normal 

 flow. 



Oedema is almost always due to the latter cause, viz. excessive 

 transudation. 



FCETAL CIRCULATION 



Certain structures are necessary to the performance of fcetal 

 circulation, but are of no use after birth. They are as follows : — 



(1) Foramen ovale. — An opening between the two auricles. 

 It furnishes direct communication between them. 



(2) Ductus arteriosus. — A blood-vessel connecting the aorta 

 and pulmonary artery. 



(3) Ductus venosus. — A blood-vessel connecting the umbili- 

 cal vein and the inferior vena cava. 



(4) The placenta and umbilical cord. — By means of the pla- 

 centa the chikl is nourished and obtains oxygen. The cord is made 

 up of two arteries and one large vein protected by " Wharton's 

 jelly." The vein carries the oxj'genated blood from the placenta 

 to the foetus. The arteries carry the impure blood from the 

 foetus to the placenta. After birth these structures are of 

 no further use. It is important that the " foramen ovale " 

 should close as soon as the child breathes, else if the arterial 

 and venous blood continue to mix, a " blue baby " will be seen. 



