Of Cutaneous Absorption. 1 199 circumstances the animal re-acquired the weight which it had before lost, and its limbs and body regained their plumpness and former volume. Dr. Edwards confined a snake in air saturated with moisture, removing it and weighing it at intervals: at first it was found to lose in weight; after a time it ceased to become lighter, and was observed to gain in weight. M. Seguin observed that when the human body is immersed in water at a temperature between 12°.5 and 22°.5 Cent, no loss of weight takes place beyond the usual loss by pulmonary transpiration. Immersion therefore in water at the above temperature, should either prevent cutaneous transpiration, or allow an absorption to take place equal to the loss it occasions. The preceding analogies are in favour of the latter solution. But subsequent researches by the same author seem to show that water in contact with the cuticle of the human body is not absorbed. If the water hold a salt of mercury in solution, it very rarely happens that any evidence of the absorption of the mineral manifests itself even after long and repeated immersion. The cuticle appears to be the main impediment to cutaneous absorption: if this membrane be removed, absorption takes place rapidly from the surface of the cutis; or if by continued pressure, as during mercurial friction, a substance be mechanically forced through it, absorption does not fail to take place; or if a substance which is of an acrid nature, and calculated chemically to combine with it, be placed in contact with the epidermis, the same result is found to ensue. * De l'Influence, &c. p. 347. 200 Cutaneous Transpiration With respect to the vessels which minister to this function, there is no more reason to believe that the lymphatics absorb poisons or medicines applied to the skin, than that the lacteals have this office in the small intestines. It cannot be doubted that when the impediment which the cuticle offers is removed or overcome, foreign matter in contact with the skin finds its way into the blood-vessels by physical transudation or imbibition. The action of the skin upon the air is obscurely understood: analogy perhaps would lead us to suppose that an absorption of oxygen takes place at the surface of the body; for the experiments of Mr. Cruickshank and of Mr. Abernethy have shown that carbonic acid is produced when the hand or the foot is confined in atmospheric air. But this subject requires to be yet further elucidated. A certain quantity of fluid continually transudes through the skin; sometimes it wholly disappears by evaporation: at other times it collects as a liquid upon the surface of the body. In the former case it is termed the Insensible Perspiration; in the latter the Sensible Perspiration. When collected the perspiration appears to consist of water containing a small proportion of acetic or lactic acid, of muriates of potash and soda, with a trace of animal matter apparently gelatin. The most unexceptionable experiments perhaps relating to the quantity of the insensible perspiration are those of Lavoisier and Seguin. Upon their testimony the average quantity amounts to eleven grains per minute. During digestion the quantity of cutaneous transpiration appears to be at its minimum. According to referable to Secretion and to Transudation. 201 Dr. Edwards, during the six hours before noon the insensible transpiration,cæteris paribus, attains its maximum. Sleep would seem to promote it remarkably: a dry state of the atmosphere, exposure to a current of air, diminished barometrical pressure, have a similar tendency. The influences last named are such as would affect the rate of evaporation from a dead body. Dr. Edwards has founded upon these and similar observations, an apparently just division of the elements of the insensible perspiration into such as are derived from secretion, and such as result physically from the evaporation of the moisture of the skin itself. Upon estimating the comparative loss of weight which frogs suffer when placed at a low temperature in dry air and in air laden with moisture, the proportion of fluid lost by secretion to that lost by transudation appeared to be as 1 : 6.d But it is possible that in dry air the quantity of secretion may be greater than in air laden with moisture; the increase of the demand may increase the quantity of the supply, agreeably with a fact respecting the secretion of milk, to which I have already adverted. At an elevated temperature and during violent exercise the perspiration becomes sensible. No estimate appears to have been made of the actual quantity of liquid produced under these circumstances, or of the ratio in which the different causes alluded to influence the secretion of the sweat. Sir C. Blagden remarked, that on staying for twenty minutes in a chamber heated to 1980 the perspiration was so little increased that his shirt was only damp at the end of the experiment. A d De l'Influence &c. p. 334. e Phil. Trans. vol. lxv. p. 119. 202 Of the Standard Heat of the Body. few minutes of violent exercise at a much lower temperature would have produced a copious flow from the skin. The principal object of the sensible perspiration appears to be the reduction of the temperature of the body. The present occasion, therefore, leads us again to consider the subject of vital heat. Heat, it seems, can be produced in all living beings; but while in plants and cold-blooded animals the temperature closely follows that of the media in which they are immersed, in mammiferous animals and in birds a given temperature is sustained, which is termed their standard heat. In human beings the standard heat is about 97°, in viviparous quadrupeds 100° or 101°: the temperature of birds is yet higher,and rises to 107° or 108°. Extremes of heat or cold appear temporarily to raise or lower the temperature of the body. After staying sixteen minutes in a dry air at 64° Cent, M. Delaroche observed the temperature of the skin to be raised 4o. During various disorders the temperature of human beings is liable to be raised to a higher standard. In fever the heat has been observed at 104°f. M. Prévost witnessed a case of tetanus in which the temperature was elevated 7° Cent above the natural standard. Mr. Caesar Hawkins mentioned to me having witnessed, that in a person who died within twenty-four hours after an injury of the spinal chord at the lower part of the neck, which crushed it, and produced paraplegia, the thermometer applied to the groin ten minutes before death rose to 111°. The young of warm-blooded animals have a tempera f Currie's Reports, p. 21 et seq. Differences observed in very young Animals. 203 ture lower than that of adults. The same difference has been noticed in the human species. M. Breschet ascertained, upon an examination of ten infants within fortyeight hours after birth, that their temperature varied from 34° to 35°.5 Cent. F There appears to be a remarkable difference in the young of warm-blooded animals as to their power of producing heat. A Guinea pig soon after birth is able to resist a low temperature nearly as well as an adult; but kittens and puppies newly-born lose their temperature rapidly when the external heat is artificially lowered; in a fortnight, however, they acquire the power of evolving heat. This difference bears a relation to the general forwardness of animals. Those which are born with their eyes open, can sustain themselves at a given temperature: the opposite class resemble at first coldblooded animals, and their temperature falls with that of the surrounding media. A parallel difference is observed in birds, some of which quickly walk and run upon breaking the egg: but others, as for instance the jay, appear hatched before their time, and three or four weeks elapse before they can sustain a standard temperature. Dr. Edwards, from whose valuable work on the influence of physical agents upon the animal œconomy I have largely borrowed, connects with the preceding remarks an interesting observation of the temperature of a child born at seven months. At this period the existence of the membrana pupillaris ranks the infant with those animals born with closed eyelids; and the temperature of the infant in the case alluded to did not exceed 32° Cent, although the child was well wrapped up and placed before a fire. |