The child, while in the womb, is in reality a part of the mother, receiving its nourishment from her and existing in a kind of passive state. The various organs of the body are in a state of complete repose, becoming prepared however for the duties they will have to perform when this passive state is over.

The muscles, the bones, the kidneys, liver, lungs, bowels, and the whole nervous system having no duties to perform, remain in a state of almost complete repose. But when the child is ushered into the world, it immediately commences a new existence. Now every organ is aroused and called into action. The lungs move, the bowels act, the kidneys perform their duty, the nervous system begins to show signs of action, and the whole machinery of life moves on in harmony.

Before, surrounded by a fluid of unvarying warmth, and nourished by the mother's blood; now, introduced into a colder and ever-changing atmosphere and wrapped in clothing, however soft it may be, still subjecting it to much harder pressure than it ever before sustained, and compelled to digest its own food and throw out its own waste, the change is speedy, and the effect on the system necessarily great. The very suddenness of the change is necessary to call into action the organs of the body, and thus secure the life of the child. If respiration is not established before the maternal circulation ceases, the child dies as if from suffocation.

The child is of course exceedingly sensitive, and the shock produced by the sudden transition of temperature from 98° to 100° in the mother's womb, to 65° or 75° in the atmosphere, calls into activity respiration. The effect produced is similar to the panting and sighing, which almost every one has noticed on plunging into a cold bath. The sensation to the child of this sudden change of temperature is so disagreeable, that generally the first intimation the mother has that her child is living, is a busty cry, and none but the mother can tell the gushing love which that cry awakens in her heart, or its quick pulsations of joy.

The extreme sensitiveness of the child to external influences is its greatest safeguard. Changes of temperature, hard or harsh clothing, want of cleanliness, and errors in food, may be the means, however trifling they might appear to the more hardy frame, of producing local disease. Hence the child gives utterance to its complaint in a warning cry of distress, the only way in which it can make its complaints known.

We have seen in a preceding chapter, that aside from a venous and arterial circulation, by which the blood is conveyed, charged with nutrition to every part of the system and returned back to the heart charged with impurities, there is a pulmonary circulation, by which the dark impure venous blood is thrown into the lungs from the heart, and there, changed by the action of the air in the air-cells, having received the elements of nutrition in the chyle, conveyed through the thoracic duct, becomes arterialized, and is thrown into the right side of the heart, and from thence conveyed to every part of the system.

In the unborn child the pulmonary circulation does not exist. The arterial blood is received direct from the mother, and as respiration does not exist, there is no necessity of the blood passing through the lungs. It therefore passes directly through, from the right to the left auricle of the heart, by what is called the foramen ovale, or oval hole. With the first gasp of the child on emerging into the air, the muscles of respiration begin to act, the blood passes into the lungs, and a new circulation commences. The foramen ovale, no longer of any use, gradually closes up, and the whole volume of blood passes through the lungs. Heretofore the child has lived through the mother, its arterial and venous circulation carried on through the umbilical cord, but now it must breathe and eat for itself. The lungs and heart are at first small, but they continue to expand and increase in size from year to year.

Partly on account of the small size of the chest, and partly from the extreme nervous sensibility, the circulation of the blood in infancy is much more rapid than later in life. While in an adult the heart contracts and the pulse beats from 60 to 80 in a minute, during the first months of life it is nearly double that number, and varies from 120 to 130. Hence we should be on our guard about mistaking a perfectly natural for a feverish pulse.

It is very obvious that the rapid circulation of the infant and its quick respiration, render it more liable, by increasing the nervous excitability, to various forms of acute diseases, than at a more advanced age, where the circulation and respiration are less rapid. Therefore an even temperature and a pure atmosphere are absolutely essential to the health of the child.

Another condition indispensable to the life of the child is the supply of animal heat. This, before birth has been obtained through the mother, but now the child occupies a comparatively independent position, and the preparation of animal heat must be carried on in its own body and by its own organs. The evolution of animal heat we have fully explained in a preceding chapter. It is the same in the child as in the adult. It depends upon respiration, digestion, and nervous excitement. The power of generating animal heat, contrary to what is supposed by many, is far less in the young than in the adult. The reasons are very obvious. The lungs are small, therefore respiration is less full and active; the diet consists of watery and unstimulating milk, and the infant is of course prevented by weakness from making use of much exertion; and lastly the nervous system is not fully aroused, the child, during the first months of its life, sleeping a large portion of the time. A temperature of about 98 is essential to man, and this we find he has, if in a state of health, whether he inhabits a land where the thermometer is 100° above zero or 20° below. This uniform temperature of the body is much warmer as a general thing than the surrounding atmosphere, and unless the internal fires were kept burning, the body would soon cool down to the same temperature as the medium by which it is surrounded. The power of generating animal heat is smaller in early life, than at any other period of existence, and therefore the power of resisting external cold is far less at that period. The pernicious effects of highly heated and impure air, and allowing the limbs of the child, both in the house and out of doors, to be destitute in a great measure of clothing, will very readily be perceived by all.