the shell, which is the porous septum through which the exchange of gases is effected, be varnished, the inhabitant will perish by asphyxia.

Respiratory activity of mammalian fœtus. Blood-gas analysis.The most obvious of the functions of the placenta is respiratory : blood passing from mother to child by the umbilical vein is of brighter colour than blood passing from child to mother by the umbilical arteries. And further insight has been obtained into the process by the gasanalysis of the blood of sheep-embryos, which at full term weigh about as much as human embryos. The following values were obtained by Zuntz and Cohnstein:

CO2 02
2.3 vols. per 100
6.3

Lamb at full term weighing 3.6 kg. umbilical artery. 47 umbilical vein 40.5

the rate of blood-flow was about 44 c.c. per minute, i.e. carrying 1.76 c.c. O, and 2.86 c.c. CO2, i.e. the respiratory activity of the fœtus was less than that of a normal adult sheep.

2

The blood-pressure in the same instance was between 6 and 8 cm. Hg in the artery, and half that value in the vein, thus illustrating the fact that the difference between arterial and venous blood-pressure is much greater in the adult than in the fœtus.

An important peculiarity exhibited by fœtal blood, and referable to the fact that such blood contains an abundance of active cellular elements (nucleated red corpuscles), is the much greater rapidity of respiratory changes occurring in the blood itself; in a given volume of fœtal blood a larger quantity of O2 is consumed and of CO, is produced than in an equal quantity of adult blood. Also, in relation to fœtal respiration, it should be mentioned that the amount of hæmoglobin in the body is at first much below that of the adult, and that it gradually approximates to it in the course of pregnancy; at half-time the relation of hæmoglobin to body-weight is only, at full time, in the adult

(egg and hen); in an early sheep-foetus the hæmoglobin value may be onlyth that of the adult; at term it amounts to . From which we learn that the respiratory function develops slowly. At birth it undergoes a sudden change-in the sheep it is increased tenfold; from which we learn further, that the conditions of active respiration are already present at a time when, for want of excitant, the process itself is still smouldering at low intensity.

The tissues of an embryo produce far less heat than those of the adult; there is no such thing as thermotaxis during embryo-life, the temperature of a fœtus is only one or two tenths of a degree above that of the mother; the heat of the embryo is carried off by the uterine circulation, precisely as heat produced by the liver is carried off by the hepatic circulation; this low production of heat is in correspondence with the low respiratory activity and with the tenacity of life exhibited by an embryo deprived of oxygen; an embryo is slowly asphyxiated in utero. if the placental circulation is arrested, it is rapidly asphyxiated by

asphyxia of the mother with intact placental circulation; in the former case embryonic tissue slowly consumes the oxygen it possesses, in the latter case the maternal tissue rapidly consumes oxygen and withdraws it from the foetus.

It is probable that the nitrogen metabolism is also of much lower intensity in the foetus than in the adult; to the question as to its mode of elimination by a fœtus in utero, it may be answered that although towards the end of the pregnancy the kidneys are evidently beginning to act, it is probable that the placenta is the main organ of urinary excretion during the period of pregnancy. The bladder of a new-born fœtus is sometimes empty, sometimes full; the amniotic fluid contains only a small proportion of urea; cases are on record of a fœtus born at full term and possessing no kidneys. These are the main facts upon which the answer is based.

The entire nutrition of the fœtus after an early period is, in fact, dependent upon the placenta; the amniotic fluid, to which nutritive action has been attributed, may, indeed, slightly contribute in this direction, but in very minor degree; it contains very little albumin and no carbohydrate or fat; whereas the placenta is a highly glycogenic organ, and has also been asserted to contain peptone.

That the amniotic fluid is of maternal rather than of fœtal origin is clearly shown by Zuntz's experiments; after injection of indigocarmine into the vessels of a pregnant sheep, he found the placenta and amniotic fluid coloured blue, without extension of colour to the fœtus itself, or even after destruction of the fœtus. On the other hand, after direct injection of carmine into the foetus he found its kidneys blued, which was proof that the organs, even if not ordinarily in action, yet are evidently capable of excretory activity.

With regard to the existence and digestive activity of embryonic ferments-ptyalin, pepsin, pancreatin-artificial digestion-experiments have furnished no very precise results, either positive or negative; or, rather, both positive and negative results have been reported by dif ferent observers. There can be no doubt that the digestive action of fœtal glands is, at any rate, much less constant and pronounced than that of adult glands, but it is probable that the ferments do already exist in the foetal organs in their zymogen state.

By far the most important of foetal organs is the liver; beginning at about mid-pregnancy, or even sooner, the excretion of bile and its accumulation in the meconium are unmistakable tokens that important chemical events are taking place in the liver; so also is the fact, alluded to above, that the liver is the chief seat of the oxygen consumption by the fœtus.

The foetal pulse can be felt in the umbilical cord for a short time after delivery, but with the closing up of the umbilical artery it vanishes. The first respiration of a new-born infant occurs in response to the combined effect upon the spinal bulb of the increasing venosity

of the blood following the arrest of the placental respiration, and of cutaneous stimuli by cold air, or it may be by the flip of a wet towel.

In normal parturition, the birth of the child precedes that of the placenta, and after a shorter or longer interval the cord is ligatured and divided. It makes a great difference to the child whether this be done immediately or some time after birth; in the latter case uterine pressure squeezes blood from placenta to child, and on the average the difference so effected is from 50 to 100 c.c. A normal child of 3 kg. should have 300 c.c. of blood, a child after immediate ligature may have as little as 200 c.c. The fact has been verified on the human subject; a child placed on a balance shortly after delivery, and left connected with the placenta in utero, gradually increases in weight by about 50 grammes.

The chick's heart, from the fourth day of incubation onwards, is capable of being submitted to certain observations and experiments; the contraction, which is at first at very irregular intervals, sweeps over the tube in a peristaltic manner from the venous to the arterial end, and by means of photography its rate of progress has been determined to be 5 to 10 mm. per sec. (Fano). Placed in a watch-glass in a few drops of fluid, it forms an exceedingly sensitive reagent to the action of nutrient or non-nutrient fluids; the beats die out in salt solution, and may be renewed by serum-albumin; examined in a gas-chamber the beats rapidly disappear with carbon dioxide and reappear under the influence of oxygen; and attention may be drawn to the fact that there is here no possible complication by action on nerve-fibres or on ganglioncells, which do not yet exist. Submitted to tetanising currents, the embryonic, unlike the adult heart, is completely tetanised. Muscarin, which promptly abolishes the beat of an adult heart, has little or no action upon the embryonic heart.

100

According to Hermann and v. Gendre, the developing chick's embryo may exhibit an E.M.F. of To Dan., any part of the dorsal surface of the embryo being positive to any point of the yolk; thus, in the embryo itself the current is directed from ventral to dorsal surface.

Intra-uterine movements of the embryo may occur as early as the seventh week, although the obvious movements known to pregnant women as quickening' are not usually noticed before the sixteenth week. Such movements are not voluntary but reflex, the result of internal changes in the composition of the blood or of external stimuli; according to Soltmann, the cortex cerebri of new-born puppies is inexcitable, and, as was mentioned on p. 488, the cortico-pyramidal system in man is of post-embryonic development. Anencephalous monsters, i.e. beings born without cerebrum or cerebellum, can perform reflex movements, and may even automatically' breathe, suck, cry, and swallow, through the agency of bulbo-spinal centres.

II.

CONSTITUTIONAL FORMULÆ OF SOME OF THE
CHIEF PROXIMATE PRINCIPLES

THE chemical relations of several of the more important substances mentioned in Chapters V. and VI., such as the fats and many of the nitrogen compounds, will be better appreciated by considering their constitutional or linkage-formulæ, and the position that these formula occupy in well known series. This point of view is particularly serviceable in the case of urea, glycin, sarcosin, taurin, leucin, and tyrosin, the constitutional formulæ of which can be given on a satisfactory basis; but it is not without value in the case of other complicated substances represented in a more or less hypothetical form, such as uric acid, hippuric acid, creatin, creatinin, indican (so-called), indol, skatol, lecithin, cystin; and the fats can be usefully so represented. As for the proteid group, and substances such as hæmoglobin, bilepigments, &c., their graphic formulæ must be recognised as imaginary pictures containing radicles grouped so as to show possibilities, and arranged to suit a very scanty knowledge of facts.

The elements we have to deal with are hydrogen, oxygen, nitrogen, carbon, and occasionally phosphorus and sulphur.

The chief radicles entering into the composition of bodies that we have to consider are: hydroxyl, HO; amidogen, NH,; the hydrocarbon radicles, CH3, C2H5, C3H7, CзH2, &c.; and the benzene radicles, C6H5, C6H., &c.

We shall best form an idea of their combinations by considering(1) Some simple linkage-formulæ of familiar bodies, H2O, NH3, CO2, HNO3, H2SO4, H3PO4.

(2) The linkage-formulæ of a simple fatty acid series, of the combination of some of its members with the glycerin radicle, C3H5, as fat, and of two or three organic compounds with nitrogen, which from the chemical point of view fall into place on the lines of series running parallel with a simple fatty acid series.

(3) Linkage-formula that include a benzene nucleus, CH5.

(4) Linkage-formulæ of a complicated or not definitely settled type.

The ordinary or empirical formulæ of water, ammonia, &c., translated into linked-formulæ, are as follows:

The formula of the hypothetical compound carbonic acid, H2CO3which although not known as a free compound, but only as H2O+CO 2, is indicated as a rational combination by the existence of corresponding salts, Na2CO3, K,CO3, &c.—serves as the starting-point of the series of fatty acids, and the type upon which urea is constructed. Its linked

formula is Ho-co; that of urea is NH-C-o; that of acetone is

CH

но

CH, CO; and substituting H for one of the hydroxyl radicles, HO, we

H

have но-c-o, or formic acid, which is the starting-point of the fatty acid series, C,H2n+1. CO.OH, where CH2, or a multiple of CH2, is the difference between different members of the series.

H

Formic acid. CH,O, or HO-C=0 or H. CO.OH.

C15H31.

Palmitic acid. C,H,,O, or 15(CH) or C1H11. CO.OH.

C16H32O2

HO-C=0

H

Stearic acid.

C18H36O2 or 17(CH)
но-с=0

or C17H35. CO.OH.