*Sympathetic Ganglia and Nerves.*—The sympathetic ganglia are found in different parts of the body, and vary in size from those which are half an inch in diameter to those that are smaller than the heads of pins. The largest and most important ones are found in two chains which lie in front, and a little to either side, of the spinal column, and extend from the neck to the region of the pelvis (Figs. 125 and 133). The number of ganglia in each of these chains is about twenty-four. They are connected on either side by the right and left sympathetic nerves which extend vertically from ganglion to ganglion. In addition to the ganglia forming these chains, important ones are found in the head (outside of the cranial cavity) and in the plexuses of the thorax and the abdomen. The sympathetic ganglia receive nerves from the central division of the nervous system, but connect with glands, blood vessels, and the intestinal walls through fibers from their own cell-bodies. Some of these latter fibers join the spinal nerves, and some blend with each other to form small sympathetic nerves. *Protection of Brain and Spinal Cord.*—On account of their delicate structure, the brain and spinal cord require the most complete protection. In the first place, they are surrounded by the bones of the head and spinal column; these not only shield them from the direct effects of physical force, but by their peculiar construction prevent, to a large degree, the passage of jars and shocks to the parts within. In the second place, they are surrounded by three separate membranes, as follows: 1. The _dura_, or dura mater, a thick, dense, and tough membrane which lines the bony cavities and forms supporting partitions. 2. The _pia_, or pia mater, a thin, delicate membrane, containing numerous blood vessels, that covers the surface of the brain and cord. 3. The _arachnoid_, a membrane of loose texture, that lies between the dura and the pin. Finally, within the spaces of the arachnoid is a lymph-like liquid which completely envelops the brain and the cord, and which, by serving as a watery cushion, protects them from jars and shocks. Thus the brain and cord are directly shielded by bones, by membranes, and by the liquid which surrounds them. They are also protected from jars resulting from the movements of the body by the general elasticity of the skeleton.