From this view it might at first be con cluded that, in order to trace the changes that ensue between the commencement of extra uterine life and the attainment of maturity, we have only to look for them in the organs of the relative life. But the survey that we are about to take of the changes in question will show that the other class of organs are by no means exempt from alteration, although the changes are not those of development. They will be found to have reference to degree or amount of function rather than to capacity.
The external characters of the infant just eliminated from the uterus at the full period of gestation are as follows : —the integuments are thin, tender, and covered with a white unctuous matter ; the nails just reach the ends of the fingers ; the trunk and limbs are round and plump ; and the articulations are in a state of flexion. The average weight of the body is about six or seven pounds ; the length varies from seventeen to twenty-one inches, sometimes falling short of or exceeding these limits. The point which lies midway between the two extremities is at the umbilicus. The dimensions of the head and of the abdomen are very large in proportion to the other cavities, and as compared with their own measurements in after periods of life. The pelvis looks con tracted, the thorax flattened at its sides and prominent in front, and the lower extremities are less developed than the upper. A line drawn from the occiput to the chin measures five inches and three lines ; from the occiput to the forehead four inches and three lines ; and from the vertex to the base of the skull three inches and six lines. The circumference of the head, taken along the course of the median line, is from thirteen to fourteen inches ; but taken horizontally, and passing over the parietal protuberances, it seldom measures more than ten or eleven inches. The contrast between this general aspect and that of a full grown man is too obvious to require any repre sentation of it here.
The characters of the interior will be best described and understood by examining ana lytically the several apparatuses of the func tions. Of the latter the most simple and primitive is assimilation, consisting of certain molecular motions which maintain, repair, and mould the organic tissues. We have already observed that the requisites for this function are perfect in the new-born infant; a copious supply of the fluid from which the textural particles are to be elaborated, a ready ingress for this fluid, and a no less ready egress for that which receives the particles no longer required in the process. All that we know
of the mechanism employed is a porous ex tensile substance, varying in its chemical con stitution according to the nature of the tissue. Porosity is resolvable into a collection of infinitely minute tubes, and the degree of porosity is, therefore, determined by the number of the tubes ; the extensibility depends on the composition of the tubes. The tissues of the infant are soft, they abound in fluids, and are more capable of imbibition or artificial injection than at later periods of life ; this being consequently possesses a complete me chanism of nutrition. But this mechanism can be of little utility unless the nutrient fluid be supplied liberally, and after furnish ing the atoms for the formation of the several textures give place to fresh supplies. These conditions are afforded by the arteries and veins.
There is no period of human existence in which the processes of interstitial growth are so active as in infancy, whether they be instanced in the accretion of matter, in the change of composition, or in the modification of form. This fact is in harmony with the state of the capillary system just described, and it will be found to correspond no less with the relative construction of the arteries and veins. The function of the former of these is to convey the blood into the tissue, of the latter to take it away ; consequently in a part where the growth is most energetic, we might, a priori, expect that the former would be more numerous, capacious, and distensible. This is well known to be the case from actual observation, partly of the effects of artificial injection, and partly of the phenomena of disease. An examination of the textural properties of the two sets of vessels leads to the same conclusion. Sir Clifton Wintringham, in his Experimental Enquiry, fully demonstrated that the venous coats in the young animal far exceed the arterial in density, and that, consequently, they are less subject to distension. When maturity is attained, the disproportion between the resistances of these vessels no longer exists.