92 



(though differently obtained) in the after-periods of life to the 

 continuance of vital processes, and from the extinction of life 

 by a degree of cold which the internal calorific powers are inade- 

 quate to resist, that the influence of heat is truly essential to these 

 processes: and if this is a genuine relation between heat and the 

 agent of life, we appear justified in assigning such relation during 

 all the stages of the uterine ovum. 



67. In addition to what we observe of the actual relation of 

 heat with life in the maturer periods of human existence, we have 

 the evidence of analogy tending to the same point, derived from 

 the crustaceous ova, whose processes of life, as above stated, are 

 found not to commence without the influence of a certain degree 

 of heat; and if this analogy is true, to the extent upon which it 

 might apparently be presumed, the following view, in regard to 

 the human ovum, will appear to receive its sanction. 



68. The fecundated human ovum differs from the crusta- 

 ceous ovum in the identical properties which compose it, and in 

 the following circumstances: 1st, the crustaceous ovum contains 

 its material of growth within itself, while the human ovum is 

 dependent for the same upon a maternal source; 2nd, the crusta- 

 ceous ovum, fecundated and prepared for the changes which pro- 

 duce mature organization, waits for the influence of a cause, 

 giving activity to its properties, which cause is external heat; 

 while the human fecundated ovum does not rest for want of such a 

 cause, but is immediately supplied with it, and its properties are 

 rendered active, and they begin to form the textures. Much 

 more remains to be said, under a distinct title, on the subject of 

 animal heat, and indeed on most of the topics hitherto spoken of; 

 but as much is here said as belongs exclusively to the condition 

 of the ovum. 



70. It was remarked that the fluid which the ovum obtained 

 from the mother, was either blood or convertible into blood by 

 the foetal system. The fostus, as is well known, or we might say 

 the placenta, cannot be injected through the uterine vessels. 

 Now, as the injections employed are as subtile as blood itself, it 

 is hence concluded that there is no communication of vessels 

 between the placenta and the uterus: how then (the question is 

 much hackneyed) does the foetus obtain the materials of growth? 



71. There is no continuity of vessels between the uterus and 

 the maternal portion of the placenta; the foetal portion of which 

 is connected with the uterus only through the medium of the 

 maternal portion. The maternal portion has blood brought into 

 contact with it: from this blood it absorbs some fluid, which 

 passes into the foetal portion of the placenta, and thence to the 

 fcetus. The circulation of the fcetus is from itself to the placenta, 

 and back again to the foetus: these things are well known. This 

 provision prevents the ill consequences of excessive repletion, or 

 accumulation of blood, in the fcetus, which would happen if 

 arterial blood passed freely into it by a direct communication 



