91 



condition by which we recognize it as r a living principle.- At this 

 period of foetalization, which might be called the uterine period, 

 it is our present business to say what are the agents which main- 

 tain the living condition, and the enumeration will not be tedious. 



61. It is necessary that the feel us should obtain from the 

 mother a fluid, which is either blood, or else convertible into 

 blood, by the foetal system. The office of this blood is inferred 

 in the case of the foetus in utero, in part, from the known uses of 

 it in the subsequent periods of life; but principally from the 

 consequences, such as the extinction of life, &c. in the foetus, 

 from an obstructed supply. It is hence believed that blood is the 

 external, which at once maintains the living state of the spirit, 

 and furnishes tiie materials of the structures. 



62. The foetus in utero is also necessarily exposed to an ele- 

 vated temperature; whether it is mere heat, imparted from the 

 double source, viz. from the blood, and from the uterine parieties 

 which supports the living principle in its preparatory changes; 

 or whether, in the human ovum, this effect is dependent upon cer- 

 tain congenial and adapted properties of blood; is a question 

 upon which we will not assert peremptorily, at any rate until we 

 have recollected some circumstances of analogy. 



63, The crustaceous ovum contains the substance which 

 supplies the materials of growth; so far it resembles blood in the 

 other case: this substance is not capable of commencing (although 

 it has the properties of blood) the processes of life, which are 

 begun and maintained by heat. 



64. The relations then which we are considering, in respect 

 to the crustaceous ovum, are as follow: 1st, the substance which 

 furnishes the materials of growth can neither begin the actions ncr 

 maintain the properties of life; 2nd, heat is capable of beginning 

 and maintaining these processes; 3rd, if, as is proved by many 

 familiar facts, the material, viz. that which performs the office of 

 blood, were withdrawn, then heat could not maintain the pro- 

 cesses of life; 4th, neither heat nor blood, the vital principle 

 being extinct, are capable of supporting the processes of life, or 

 of renewing them when they have ceased. 



65. It follows, therefore, from the facts in the last paragraph 

 and other preceding ones, 1st, that in the crustaceous ovum a 

 (quiescent (quiescent from the state of combination of its pro- 

 perties) organic spirit must exist in the first place; 2nd, that 

 heat makes its properties active; 3rd, that their actions are after- 

 wards preserved, and continued in a series of changes by the con. 

 joined influence, or energy, or causes, of the organic spirit, of heat, 

 and of the properties allied with the material of growth. 



66. In the uterine ovum of viviparous animals, we have more 

 difficulty in discovering what share heat has in its growth, because 

 there is not, as in the oviparous animals, a state of rest in the 

 constituents of the ovum for any length of time subsequent to 

 fecundation. But we judge, from the iudispensability of heat 



