We must not leave the alimentary tract with out observing that the fibres of the stomach and intestines in infancy and childhood are, like those of the heart and other involuntary muscles, more irritable than in after life ; hence the contents of these viscera are propelled more rapidly, and the evacuations are more frequent; their tissue is also softer, and their colour more approaching to white.
The liver undergoes a great change after birth both in form and in function. The pecu liar circulation of which it formed so important an organ during foetal life being abolished, the left lobe which nearly equalled the right in volume, is diminished to a third of its original size. But while the umbilical vein and the canalis venosus are obliterated, the vena port is developed, and the bilious secretion becomes the predominant function. Of the further changes which this organ experiences, we have very little knowledge, except that the whole bulk is greatly lessened, and that the colour of its parenchyma becomes darker, and that it is more subject to disease in after periods. Oc casionally we meet with instances in which the foetal proportions of the liver continue through life (Andral). The bile has not been carefully examined with reference to particular ages, but it is known to be less viscid and to contain a smaller quantity of its peculiar principles in infancy ; while its greater liability to con cretions at more advanced periods indicates an alteration in its composition. The gall-blad der, though small at birth, contains bile, green in colour and bitter in taste, and soon becomes enlarged.
The spleen also increases in volume, but what alteration takes place in the progress to maturity, in its function, must, of course, be doubtful until the function itself be better understood. Probably its enlargement is con nected with the distended condition of the venous system. Of the changes in the pan creas and salivary glands, we know little more than that their texture increases in firmness. The lacteals, lymphatics, and their respective ganglions have a very marked development. It is to be regretted that no observations have as yet been made upon the composition of the chyle at different ages. There are doubtless many alterations corresponding to the varying activity of the digestive function, and to the kinds of aliment used at those periods.
So much for the organs and functions which are concerned in augmenting or modifying the nutrient matter. Before proceeding to those of the relative life, we must allude to the organs of excretion. The kidneys at birth have not lost the traces of their lobular forma tion, but these are effaced. The weight
of these organs at birth is to that of the whole body as 1 80 ; in the adult 1.240. The me dullary portion is more abundant than the corti cal in early life. The supra-renal capsules soon be gin to shrink from their foetal size. The ureters are large, and the bladder has a more elongated form than in after periods ; it also occupies a higher situation above the pelvis. The func tional qualities of these forms are not so well ascertained as the analogy of their organization to that of inferior animals. The urine is retained a shorter time in the bladder ; it is more aqueous and less impregnated with saline and animal ingredients than in after life; there is also a particular deficiency of urea. Of the intes tines we have already spoken ; their contents are copious but less feculent than they after wards become. The perspiration affords a si milar character to that of the other excrementi tious secretions, being more aqueous, less sa line, and less odorous. On the whole it may be said that less activity is indicated in the egestive than in the ingestive system.
Of the defensive organs, or those which are exposed to surrounding agents, we may remark, in general terms, that although fully adequate to the demands of the infant under the circumstances of his existence, they acquire a development proportionate to his growing in dependence of the care of others. The skin increases in firmness, and the epidermis in thick ness ; the sebaceous follicles become larger and more numerous, and the hair is more abundant.
There is a portion of the nervous system which we have every reason to consider more related with the functions which have been just reviewed, than with those of the animal life, and which might a priori be expected to bear a corresponding ratio of developement. We allude to the ganglions; they appear to be fully formed at birth, but what changes they undergo between that period and maturity we do not profess to know. In old age their tissue is found hardened, shrunken, and of a greyish colour. (Bichat.) The changes that we have next to take notice of are of a totally different character from the foregoing. They consist not merely in augmentations of size, correspondently with the general increment of the body, or in modi fications of organs according to the altered circumstances under which they have to act, but in processes essential to the completeness of certain organs. These are the parts em ployed in locomotion, voice, sensation, and thought. We shall begin with the osseous system.