Body Cavities

BODY CAVITIES. Comparative anatomy and embryology. In coelenterates, as well as in flatworms, there is only one single internal cavity—the intestinal cavity—which in these cases is usually provided with outgrowths or even transitions into a peripheral system of canals (for example, in jellyfish). In flatworms, the entire space between the intestine and the integument is filled with so-called mesenchyme (parenchyma), in which lie complex reproductive organs, containing a system of cavities and canals, as well as excretory organs (protonephridia). In other worms, besides the intestinal cavity and reproductive cavities, there appear more or less clear cavities in the parenchyma surrounding these organs. For example, in roundworms, there is an extensive cavity between the intestinal wall and the outer body wall. This cavity has no epithelial lining, represents the result of the divergence of mesenchymal cells, and is called the primary body cavity. In higher worms (especially in annelids), there is also a cavity between the intestine and the body wall, but this cavity, usually divided into right and left halves, and also connected with the general segmentation of the body into a series of successive chambers, is lined with flat epithelium and has a quite definite connection with the gonads, which develop in its walls, and with the excretory organs. This is the secondary body cavity, or coelom. Comparison with acoelomate and pseudocoelomate worms shows that the secondary body cavity is the result of the proliferation of the cavities of the gonads (the gonocoel theory of Hatschek) (Fig. 1). The se-

Figure 1. Diagrams of the formation of the segmented secondary body cavity of a higher worm from the reproductive parts of a lower worm: 1- foregut; 2- gonads; 3- protonephridia; 4- hindgut; 5- nephridium; 6- secondary body cavity.

condary body cavity is well developed not only in annelids but also in enteropneusts, as well as in echinoderms. In mollusks, it is poorly developed, appearing mainly in the form of the pericardial cavity, and in arthropods, it is reduced and enters into connection with the circulatory system. The secondary body cavity acquires great importance in all chordates and in particular in all vertebrate animals, where it reaches a particularly complex differentiation. Embryologically, the secondary body cavity does not develop in the same way in all animals, and this has given rise to a divergence in views on the origin of the cavity itself. Sometimes it develops by the divergence of cells ("schizocoel") of the mesodermal bands growing out from the region of the blastopore lips in a young embryo (annelids, mollusks); in other cases, it develops simultaneously with the development of the mesoderm itself by separating from the primitive intestinal cavity. This latter, "enterocoelic" method of mesoderm and secondary body cavity formation is observed in some invertebrates (in chaetognaths, enteropneusts, and echinoderms) and in chordates, although in vertebrates proper, this process is somewhat modified and therefore not as clear. These observations have led to the conclusion that the secondary body cavity is the result of the separation of peripheral parts (outgrowths) of the intestinal cavity (the enterocoel theory of the Hertwig brothers). Since, however, comparative anatomical data provide definite indications of the primary reproductive function of the coelom, the previously mentioned gonocoel theory currently has more supporters. The secondary body cavity, always paired in origin, can have a metameric structure, being subdivided into a series of successive chambers, the contacting walls of which form transverse septa or dissepiments. The inner walls of the right and left sections of the body cavity cover the intestine from the outside and fuse both above and below it, forming dorsal and ventral mesenteries, upon which the intestine is suspended within the coelom. The inner wall of the secondary body cavity appears in the form of the intestinal visceral layer, or splanchnopleure; the outer wall is called the cutaneous, parietal layer, or somatopleure. At the expense of the latter, a significant part of the animal's musculature develops. In the walls of the coelom, as mentioned, the gonads develop, and its cavity communicates in higher worms with the external environment by means of segmental canals serving as excretory organs (metanephridia), and by means of the same series of shorter funnel-shaped canals serving to discharge mature reproductive products. In the lancelet (Amphioxus), the coelom develops by the separation of a whole series of paired sac-like outgrowths of the primitive gut (Fig. 2). The secondary body cavity is then subdivided into a dorsal section—the cavity of the muscle segment (myocoel)—and a ventral section—the cavity of the lateral plate or the definitive body cavity (splanchnocoel). The latter loses its metamerism, as the dissepiments disappear, and thus continuous right and left develop

Figure 2. Formation of primary segments and body cavity in the lancelet (transverse sections through the body of the embryo): 1- ectoblast; 2- neural plate; 3- mesoblast; 4- notochord primordium; 5- entoblast; 6- intestinal cavity; 7- coelom.

body cavities, which then, due to the partial disappearance of the mesenteries (especially the ventral one), merge into one unpaired cavity. In some cases, cavities of ectodermal origin develop in animals, encompassing their bodies to a greater or lesser extent. Such is, for example, the peribranchial, or cloacal, cavity of tunicates, through which the respiratory current of water is discharged, feces and excretions are removed, and reproductive products are also discharged (eggs often undergo a more or less significant part of their development in it). The "epicoel" developing from the outer integuments of the body certainly has nothing in common with true body cavities. In vertebrates, the general body cavity develops more or less independently of the intestinal cavity, but traces of its initial connection often remain, and in any case, the mesodermal walls limiting it develop in connection with the entoderm (Fig. 3). The general body cavity of vertebrates is laid down as continuous, unsegmented, and initially has the appearance of a narrow slit between both layers of the mesoderm. Later, the dorsal part of the mesoderm, and consequently the cavity contained within it, segments, forming a series of primary segments, or somites, with small cavities connected by narrow canals in the "somite stalks" with the more extensive mesoderm and body cavity of the amphibian (transverse section through a young

Figure 3. Formation of the mesoderm and body cavity

embryo of a newt): 1- primitive mouth; 2- ectoblast; 3- parietal mesoblast; 4- visceral mesoblast; 5- cavity of the primitive gut; 6- entoblast. paired cavity of the unsegmented lateral plate (Fig. 4). The latter grows along the sides of the intestine until it meets above and below the intestine, where the visceral layers of the right and left body cavities, connecting with each other, form the already mentioned mesenteries. The part intermediate between the somite and the lateral plate, specifically the somite stalk, gives rise to the renal tubules, which in vertebrates also initially open with funnels into the secondary body cavity. Along the sides of the dorsal mesentery, a paired thickening of the dorsal wall of the lateral plate develops, representing the primordia of the gonads. The lateral plates themselves with their cavities give rise to the body cavity with its peritoneal lining. The ventral mesentery

mostly disappears, and both cavities are thus connected into one unpaired body cavity. The part

Figure 4. Differentiation of the mesoderm and formation of the body cavity in a mammal (transverse section through the 6th somite of a young dog embryo according to Bonnet): 1- neural groove; 2- primitive vertebra; 3- urogenital plate; 4- primary segment; 5- epidermal layer; 6- entoderm; 7- coelom; 8- primary aorta; 9- chordal plate.

lying adjacent to the intestine, the part of the visceral layer, gives rise, in addition to its peritoneal lining proper (through the process of mesenchyme or "coelenchyma" formation), also to the connective tissue of its mucous membrane, blood and lymphatic vessels, and intestinal musculature. The somatopleure gives rise, in addition to the peritoneum, to numerous connective tissue elements. The main longitudinal blood trunks, together with the heart, also develop at the expense of the same mesenchyme in the form of longitudinal canals in the ventral and dorsal mesentery.

In adult vertebrates, the general body cavity is lined with squamous epithelium and encompasses the intestinal canal (with the stomach and esophagus) and its glands (liver, pancreas), the heart with a number of large vessels, and the reproductive organs. From the internal (visceral) walls of the body cavity, the serous lining of the intestinal tube and other internal organs is preserved in the adult animal, as is the dorsal mesentery, which suspends the intestine from the dorsal wall of the cavity in the form of a double fold. Of the ventral mesentery, only insignificant remnants are preserved in the region of the liver (lig. suspensorium hepatis). Other internal organs (glands, reproductive organs) also hang inside the body cavity on double folds of the peritoneum (Fig. 5). The parietal layer forms the lining of the outer wall of the body cavity. In all vertebrates, the anterior part of the body cavity, which contains the heart (pericardial cavity), is separated from the posterior part, or abdominal cavity. In lower vertebrates, a connection between these sections is still preserved by means of special canals. The lining of the pericardial cavity is called the pericardial sac or pericardium.

The pericardial cavity, located initially in front of the abdominal one, moves backward in terrestrial vertebrates and presses into the abdominal cavity, from which it is separated again in mammals with the formation of a transverse partition—the diaphragm. The latter develops from several sources. In all vertebrates, on the ventral side, between the venous sinus of the heart and the stomach, a transverse fold of the peritoneum (septum transversum) develops in connection with a pair of large venous trunks (Cuvierian ducts). In mammals, a pair of dorsal folds also develops, which, through connection with the ventral one, gives rise to the diaphragm. Somatic myoblasts penetrate into these folds, subsequently transforming into muscle fibers. In the thoracic cavity thus separated, the heart, surrounded by the pericardial cavity (lined with the pericardium), is located in the middle, and the lungs, surrounded by pleural cavities with their lining (pleura), are located on the sides. The esophagus passes between both lungs from the dorsal side.

I. Schmalhausen. Pathological changes in body cavities may consist of disturbances in their shape and configuration, changes in volume, accumulations of various pathological products, etc. Disturbance of the shape and configuration of one or another body cavity most often occurs during various displacements, as well as changes in the volume of organs and tissues bordering the body cavity; various pathological growths originating from adjacent parts can have the same result. Partial and even complete destruction of a body cavity is usually the consequence of connective tissue adhesion of the contacting walls of the body cavity, most often as a result of an inflammatory process; such partial or complete overgrowth of a body cavity is called obliteration, whereas the formation of local connective tissue adhesions is referred to as synechiae. An increase in the volume of a body cavity is most often caused by the accumulation of pathological material in the body cavity. Sometimes a body cavity is, as it were, inflated by air or gas that has penetrated into it from adjacent parts or has been introduced intentionally, which most often happens in the pleural cavities (see Pneumothorax). Often, body cavities serve as a site for the accumulation of dropsical fluid (see Ascites, Hydropericardium, Hydrothorax), which can significantly increase the volume of the body; the accumulation of inflammatory exudate in a body cavity (see Pericarditis, Pleurisy, Peritonitis), as well as the effusion of blood into a body cavity, has the same consequence. All the above-mentioned accumulations can be located freely in the body cavity, occupying the lower sections of the body cavity due to gravity (except for air, which is located in the upper sections), whereas in the presence of partial adhesions and obliterations, the accumulation is located in one or another part of the body cavity, being isolated by adhesions (encysted accumulations). Large accumulations of air or fluids in a body cavity cause significant displacements of adjacent organs. Severe deformations of a body cavity are produced by tumors, both those primarily developing from the epithelium of the serous coverings of the body cavity (rare primary cancers of the pleura, pericardium, peritoneum) and those that grow into the body cavity from adjacent organs or metastasize into it and seed the cavity to one degree or another, spreading through lymphatic vessels and by way of implantation. In the abdominal cavity, there sometimes occurs a kind of seeding with mucus, which is secreted either from a perforated vermiform appendix or an ovarian cyst (so-called pseudomyxoma peritonaei).

Parasites are sometimes found in body cavities, penetrating here most often from adjacent organs (ascarids from a perforated ulcer of the stomach or intestine, echinococcus from the liver or lung). Foreign bodies rarely enter body cavities; these include bullets and shrapnel penetrating during gunshot wounds, needles penetrating through the integument or from the gastrointestinal tract, surgical instruments, gauze sponges, and cotton balls occasionally forgotten in body cavities during operations. Bile and intestinal stones, as well as plant residues from food, sometimes enter the abdominal cavity during corresponding perforations; sometimes detached tumors of the ovary, uterus, or simply detached appendices epiploicae (sometimes petrified) play the role of foreign bodies. The term corpora libera is used to designate small, smooth nodules appearing in body cavities, most often in the abdominal cavity, consisting of fibrin and analogous to the rice bodies of joints. Upon the fixation of a foreign body to the wall of a body cavity, a process of encapsulation develops, usually with the formation of giant cells (see also Peritoneum, Pericardium, Pleura).

A. Abrikosov.