of fat with that of marrow, know this phenomenon, when they say that children have yet no marrow in the bones? This absence of medullary fat in the fœtus, essentially distinguishes the marrow from the ordinary fat, which, at this age, is already very abundant. _Functions._ The first and principal use of the medullary organ is to separate the marrow from the mass of blood by means of exhalation, for it has no glands, and afterwards carry it into it again by absorption, when it has remained for a certain time in its reservoir. This double phenomenon is very analogous to that which takes place in regard to the fat, for which we see that there are two orders of vessels distinct from the sanguineous, that enter its texture; it is not possible however to demonstrate them anatomically. Is the activity of the exhalants varied by exercise or rest, heat or cold, corpulency or emaciation? We have not any precise experiment upon this subject, though numerous conjectures have been made. But we know that in phthisis, dropsy and marasmus, and in general, in all those states of the body in which general debility is carried to an extreme by the slow and gradual loss of the forces, the marrow, like the other fluids as well as the solids, is changed, loses its essential characters, its consistence and takes an appearance wholly different, without however the medullary membrane experiencing any lesion, or being thickened. I have never observed this lesion except in rickets. The appearance of the marrow in these diseases is mucilaginous, gelatinous, similar to what is seen in the fœtus, with the difference of the redness, which is produced in the first age, by the great number of blood vessels.