With regard to the formation of fat in plants, it is worthy of obser vation that all seeds which yield oil on pressure—as the castor-oil seed, hemp-seed, &c—contain starch in their early stages, this starch disappearing as the oil increases, and when the seed is completely developed not a trace of the starch remaining. This renders it probable that theme fatty matters are formed from starch. From their ultimate composition it is obvious that whenever fats are produced from any substance there must be produced at the same time either highly oxidised compounds, or else that oxygen must be itself liberated. Liebig observes that if from the formula for starch, C,, 11,„ 0,„, we take nine equivalents of oxygen, there will remain in 100 parts— C,, . ..... . 79'4 11,„ O 9.8 The empirical formula for fat which comes nearest to this is C„ 0, which gives in 100 porta— C,, . ..... . 73.9 . ..... . 11'6 0 . . ..... .
According to this formula an equivalent of starch, in order to be converted into fat, would lose one equivalent of carbonic acid and seven of oxygen, or (expressed in symbols) C,, H,,, C„ 0 + C 0, + 0. , The same point is also clearly shown by contrasting the ultimate composition of starch and fat.
Starch. Human Fat. (Chevreul.) Carbon . . . . . . Hydrogen . . . . . Oxygen . . . 48'98 . . . As we are not acquainted with any constituent of plants which can take up the oxygen thus liberated in the formation of fat, we must regard this as one of the sources of the oxygen given off by plants. Mulder has given the following scheme as illustrative of the mode in which starch may possibly be converted into fat or oil in the vegetable kingdom As to the mode in which fat is deposited, there is reason to believe that it is immediately formed out of the blood, without any glandular apparatus for secreting it, by the capillary arteries of the adipose resides. By chemical analysis, the materials of fat, like those of all the other secretions, are found to be contained in the blood.
As diffused over the body, the adipose membrane consists of masses which vary considerably in their magnitude and shape. In some places they are rounded, in others pear-shaped, and in the median line of the abdomen, egg-shaped. The distribution of the membrane is exceedingly unequaL There is, in general, a considerable layer imme diately beneath the skin ; and especially between the skin and the abdominal muscles, where it occasionally accumulates in enormous masses. Between-the folds of the membranes which form the omentum
and mesentery there is usually a large quantity ; also around the heart and the kidneys ; on the face, and especially on the cheeks, and in the orbits of the eyes ; in the palms of the hands, the soles of the feet ; the pulp of the fingers and toes, the flexures of the joints, the fibres of muscles, and the sheath of vessels. In most of these organs it never entirely disappears, whatever be the degree of leanness to which the body may be reduced ; while in the cranium, the brain, the eye, the ear, the nose, and several other organs, there is none, what ever be the degree of corpulency.
The functions of the adipose tissue are manifold and apparent, 1. It fills up interstices, and acts as a kind of pad or cushion for the protection of organs which would be otherwise injured by the move ments of the body; so essential does, it appear in some parts that even where there is great emaciation it does not wholly disappear.
2. By its non-conducting power it assists in maintaining the heat of the body when exposed to external cold. It is found in immense quantities in the animals inhabiting the Arctic Seas, as in the whale tribe, and also in all animals living in the colder parts of the earth.
3. It acts as a storehouse for fuel during times of necessity. Some animals are exposed to a want of combustible food in the winter time, and they accordingly become fat in the autumn, and are thus supplied with niaterial for maintaining their animal heat. It is well known that fats are amongst the most important agents of food by which animal beat is maintained. Animals that hybernate depend solely for their existence upon the fat deposited in their bodies, which is gradually consumed during hybernation. 4. The presence of fat seems to favour the development of protein tissues. It is always found in the ova of animals before the embryo is formed. The administration of oils in certain diseases attended with emaciation, as in phthisis, has been found most beneficial, and appears to act favour ably, by assisting the development of protein tissues (Lehmann, Physiological Chemistry; Kiilliker, Ilandbuch der Gewehe Lehre ; Carpenter's Principles of Physiology.)