However well provided the infant may be with the mechanical apparatus of pores and vessels, these can be of no avail unless the fluid they contain possesses certain chemical properties. Now the blood in early extra uterine life presents the same general characters as in more advanced periods; but there is yet wanting a comparative analysis of this fluid at different ages.*e Inferentially we can enter tain no doubt that it is fully adapted to the purposes of nutrition, when we consider the conditions of the chylifactive and respiratory functions, and that, although the differences of its composition in early and in more mature periods have not been defined by experiment, they must bear a relation to the different de grees of nutrition and secretion. The differ ence, however, between the blood of the infant and that of the aged is perceptible to the senses, and will be noticed hereafter.
Pursuing the channels of the blood to the heart, we find this organ, as stated above, complete in its functions. Its volume, how ever, is large in proportion to the size of the body. Its parietes are less firm in texture, and of a paler colour than they afterwards become ; but their contractility is more active. The pulsations are from 120 to 140. in a minute. The large volume is in harmony with the quantity of the fluid, the comparative weakness of its parietes with the small extent to which their impulse requires to be propagated, and with the trifling resistance; and the quick successions of its contractions furnish the fresh supplies of the nutriment required by the energy of growth. In the progressive develop ment of this organ we notice that the bulk, although increasing so long as general growth continues, is proportionately smaller, a cir cumstance that corresponds with the diminution of the circulating fluid ; the fibres become stronger and of a deeper hue, so that the contractions are more capable of propelling the blood through the greater extent which it has now to traverse, or, more strictly speaking, of communicating a shock to a greater column; but the pulsations are slower, agreeably to the diminished requirements on the part of the capillary actions. We must not omit to ob serve that at birth the parietes of the left ventricle scarcely exceed those of the right in thickness ; but from this period an alteration commences, and rapidly proceeds until the thickness of the latter is to that of the former as 1 : 4. This change corresponds with the closure of the foramen ovale, the obliteration of the ductus arteriosus, and the consequent execution of the systemic circulation by the left ventricle only. The relative capacities of the right and left cavities begin to alter soon after birth. From tables given by Meckel it appears that, while at birth the capacity of the former compared with that of the latter is as 1 : 11, at the age of 50 it is nearly 3 : 1.* The lungs at the moment of birth undergo a more remarkable alteration in their form, their texture, and their contents, than any other organ in the system ; but during infancy and childhood they present no appreciable change in their organization, although a change must be inferred from the increase of their function. In infancy there is a smaller con sumption of oxygen ; and the power of gene rating heat, a function so intimately connected with respiration, is inferior to that possessed in later periods. Much light has been thrown on this subject by the researches of Dr. Ed wards; and practical observations of the highest importance in the management of infants, founded upon the facts which he has ascer tained, are to be met with in his valuable work.t The inspirations and expirations are more frequent at this early period, although the chemical actions between the air and the blood are less considerable. This greater fre quency is a necessary accommodation to the rapidity of the circulation. At puberty there is a marked development of the organs of respiration ; the volume of the lungs increases in conformity with the expansion of the thorax; while the greater determination of the blood to their vessels is indicated by the deeper hue of the parenchyma, by the liability to pulmo nary hemorrhage, so characteristic of this period, and perhaps also by certain diseases which affect the nutrition of these organs.
The corresponding activity of function is indi cated by the increased power of calorification, the energy of muscular motion, and the exalta tion of the cerebral actions; functions well known to have a direct relation with that of respiration ; while the establishment of the generative faculty appears to own a connexion, though somewhat more remote, with the pul monary development.
We pass from the system which imparts new properties to the blood to that which supplies it with nutriment. No imperfection is discoverable in the apparatus of digestion in the new-born infant ; every organ is com plete as an organ, but passes through va rious changes in adaptation on the one hand to the food that is supplied, and to the mode of receiving it, and on the other hand to the demands of the other parts of the body. The organs employed in conveying and modifying the chyle, viz. the lacteals and the mesenteric glands, are in a state of high development, as indicated both by their size and by their tendency to disease. The stomach and duo denum are fully formed, but the sensibility of their mucous membrane is adapted only to the milk of the mother ; any other kind of food has a greater or less tendency to produce irritation. This membrane is thick, extremely villous and vascular, and consequently of a rose-colour: In young persons it assumes a milky or satin-like appearance; in the adult it becomes slightly ash-coloured, especially in the duodenum and in the commencement of the ileum ; in the old subject it is more de cidedly ashy. Its whitish appearance, according to Andral,t is found either in very old persons or in younger subjects who have died of ma rasmus. In the adult the small intestines, according to Orfila,X bear a proportion of eight to one as compared with the distance from the mouth to the anus ; in the infant the propor tion is no less than twelve to one.§ The large intestines are longer with respect to the small intestines in the infant than in the adult; but their calibre is proportionally smaller. Ascending to the mouth we might be tempted to say that there is evidence of incompleteness in the ab sence of teeth; hut a moment's consideration assures us that the organs collected in this part are all eminently adapted to their function. The food is already prepared by the mother, and only needs to be extracted and conveyed into the pharynx ; actions which are perfectly achieved by the lips, cheeks, and tongue. When the period has arrived at which this food can no longer be furnished with safety to the mother, and when all the purposes are accomplished which were intended in this close connexion between the two beings— purposes in all probability of a moral as well as a physical character—the infant is prepared for a more independent existence by the emer gence of teeth. This event generally begins about the sixth or seventh month by the appear ance of the two middle incisors in the lower jaw ; these are followed by the corresponding teeth in the upper jaw ; next are seen the lateral incisors below and above : the rest appear in the following order ;—the first molars, the canines, and the second molars ; those of the lower jaw having generally the priority of emergence. The milk-teeth, as they are called, by the end of the seventh year have given way to the second and permanent series. For the different characters of the two sets, the order of their appearance, and other par ticulars, we beg to refer the reader to the article TEETH. That the first series should be only temporary is a necessary provision, in conformity with the change in the conforma tion of the maxillary bones which ensues at the same time.