In addition to these, which are the principal connexions, the common cellular membrane is united in every direction with the special di vision ; the details, however, of these commu nications belong to the descriptive anatomy of the several regions, to the articles on which the reader is referred.
The quantity of the interstitial tissue varies according to the age and temperament of the individual, and to the region of the body in which it is examined; but, independently of any original differences which exist, it is well knol,vn that the mode ofliving and habits of the individual have a great influence in this respect : thus an habitual full diet, especially if con joined with indolence, causes a great accumu lation of the cellular substance ; whilst, on the contrary, a spare or modemte diet and exercise will reduce it in a remarkable degree. These differences depend, probably, more on the accu mulation of serous fluid and on the repletion of the bloodvessels, than on the actual increase of the proper filamentous tissue : we can in this manner, and in no other, understand how, by by what in England is called training, the bulk of the body may be so rapidly diminished.
The proportion of this tissue varies also in the different regions of the body ; but as it is in an especial manner subservient to the pro duction of free motion, it is principally accu mulated in those parts which are most move able. It is on this account that it abounds on the face, especially around the globe of the eye and about the cheeks, and also on the forepart of the neck and of the trunk in general. In the limbs it is met with in considerable quantity in the flexures of the joints, in the axilla, the elbow, the wrist, and in the palm of the hand; also in the groin, in the ham, in the front of the ankle, and in the sole of the foot. The super ficial muscles, which are very moveable, are separated from each other by thicker layers of membmne than the deeper-seated and more fixed. It may also be remarked that those important organs, which are most liable by their structure or connexions to rupture or other effects of external violence, are carefully pro tected by being lodged in a large quantity of cellular substance. It is thus that we find the pancreas and the kidneys enveloped in this tissue in the abdomen; the bladder and genital organs in the pelvis ; and the bloodvessels and nerves in all parts of the body.
Of the special cellular membrane. —Each organ in the body is invested in a proper cover ing of the cellular tissue, and also receives into its interior, processes which envelope and join together its component parts.
The investing cellular membrane (t. cellu laris strictus) is united by one of its surfaces, the external, with the general cellular tissue, and by the other or internal with that entering into the organ. It presents many peculiarities as to the mode of its connexion ; the solid parts, for instance, as the glands, muscles, and nerves, are entirely surrounded by cellular envelopes; and a somewhat similar disposition is observed around the bloodvessels, lympha tics, and excretory tubes. On the contrary, the skin, the mucous and serous membranes, having one surface free or unattached, are only connected on one side with the cellular tissue, which is distinguished according to its situation, by the terms subcutaneous, submucous, and subserous cellular tissue. The covering thus afforded to each individual organ serves in a certain degree to insulate and separate it from the surrounding structures, and in this manner it often tends to limit the progress of disease; but as we have just seen that this covering is united both to the interstitial and to the pene trating cellular tissue, it would be equally con trary to reason and experience to expect that it should constitute, as some authorities have con tended that it does, a species of atmosphere around the various organs, confining their natu ral actions and morbid phenomena.
The penetrating cellular tissue ( I. cellularis stipatus) constitutes so essential a part of organized structures, that there is no organ in which it may not be detccted. It exists in the substance of bone, cartilage, and ligament, although it is distinguished in these structures with difficulty, in consequence of their great density ; it penetrates between the most minute fibres of the muscles and nerves ; between the coats of the bloodvessels and lymphatics ; also between the layers composing the skin and inuovus membranes ; and lastly, it enters into the substance of the absorbent and secreting glands, investing their several component parts.