378 GENERATION. SECT. XXXIX. i. i. 



As the want of this oxygenation of the blood is perpetual, (as 

 appears from the incefTant necefiity of breathing by lungs or 

 gills,) the vefTels become extended by the efforts of pain or defire 

 to feek this neceflary object of oxygenation, and to remove the 

 difagreeable fenfation, which that want occafions. At the fame 

 time new particles of matter are abforbed, or applied to thefe 

 extended veiTels, and they become permanently elongated, as the 

 fluid in contact with them foon lofes the oxygenous part, which 

 it at fir ft poflefled, which was owing to the introduction of air 

 along with the embryon. Thefe new blood-vefTels approach the 

 fides of the uterus, and penetrate with their fine terminations 

 into the veiTels of the mother; or adhere to them, acquiring 

 oxygene through their coats from the paffing currents of the ar- 

 terial blood of the mother. See Sed. XXXVIII. 2. 



This attachment of the placental veflels to the internal fide of 

 the uterus by their own proper efforts appears further illuftrated 

 by the many inftances of extra-uterine fetufes, which have thus 

 attached or inferted their veflels into the peritoneum , or on the 

 vifcera, ex^ftly in the fame manner as they naturally iniert or 

 attach them to the uterus. 



The abforbent veflels of the embryon continue to drink up 

 nourifhment from the fluid in which they fwim, or liquor am- 

 nii ; and which at firlt needs no previous digeitive preparation ; 

 but which, when the whole apparatus of digeftion becomes com- 

 plete, is fwallowed by the mouth into the ftomach, and being 

 mixed with faliva, gaflric juice, bile, pancreatic juice, and mucus 

 of the inteftines, becomes digefted, and leaves a recrement, 

 which produces the firfl feces of the infant, called meconium. 



The liquor amnii is fecreted into the uterus, as the fetus re- 

 quires it, and may probably be produced by the irritation of the 

 fetus as an extraneous body j fince a fimilar fluid is acquired, 

 from the peritoneum in cafes of extra-uterine geftation. The 

 young caterpillars of the gad-fly placed in the fkins of cows, and 

 the young of the ichneumon-fly placed in the backs of the cater- 

 pillars on cabbages, feem to produce their nourifhment by their 

 irritating ,the fides of their nidus. A vegetable fecretion and 

 concretion .are thus produced on oak-leaves by the gall-infect, 

 and by the cynips in the bedeguar of the rofe j and by the young 

 grafshopper on many plants, by which the animal furrounds it- 

 felf with froth. But in no circumftance is extra-uterine gefta- 

 tion fo exactly resembled as by the eggs of a fly, which are de- 

 pofited in the frontal finus of (heep and calves Thefe eggs float 

 in fome ounces of fluid collected in a thin pellicle or hydatid. 

 This bag of fluid comprefles the optic nerve on one fide, by which 

 the vifion being lefs diftinct in that eye, the animal turns in per- 

 petual 



