Curiously enough, whilst mammals have to depend entirely on pressure and exercise for anything but the slowest flow of the lymph, the cold-blooded vertebrates, fish, amphibia and reptiles (and even some birds), have remarkable, rhythmically contracting, muscular sacs, which pump the lymph from large lymph-vessels into large veins, and are called "lymphatic hearts." The eel and other fish have them in the tail, but they are best seen in the common frog. There is an anterior pair, one under each shoulder-blade, and another pair, one on each hip. Each opens at one end into a large "collecting" lymph-vessel, and at the other end into a large vein. They "beat" like a heart, but do not keep time with one another. Their muscular walls are formed by what is called "striated" muscular tissue (as are those of the blood-heart), and they are under the control of branches of the spinal nerves. The movement of the hinder pair in a frog can be seen through the skin. In man and all vertebrate animals the intestines, stomach and liver, heart and lungs (or swim-bladder) lie loose, except for a fibrous band of attachment, in a great cavity (often divided into two or more chambers), which they fit fairly closely. The small space between them and the walls of the cavity is occupied by a liquid. This is lymph, and the great cavity is a lymph-space. When this cavity is in its primitive form it is called the body cavity, or "cœlom." In man and mammals it is divided into four chief chambers--the peritoneal cavity (in which the stomach, intestine, and liver are loosely attached and have a certain mobility), the right pleural and left pleural cavity (one for each lung), and the pericardial cavity (for the heart). These great chambers are part of the lymph-system, and so is the lymph-holding space around and within the brain and spinal cord, and so are the great spaces beneath the frog's skin.