133. In the working of a nervous system in any animal, there are three sets of parts involved; namely, the nervous centre, the terminal organ, and the link of communication between the two, namely, the nerve. The distinctive part of every nervous centre consists of nucleated corpuscles, and any nervous mass containing nerve-corpuscles is called a ganglion. The nerve consists of uninterrupted fibres in structural continuity with the corpuscles, and without any branching until close to their termination; and the terminal organs are likewise in structural continuity with the nerves.

These terminal organs are, however, of very various descriptions, and with as much claim, in many instances, to be separately grouped as to be considered along with the nervous system to which they are so intimately united. For example, it would be difficult to raise a valid objection to considering muscular fibres in their entirety as terminal organs of nerves; yet they have a development and function of their own, and it would be inconvenient, as well as erroneous, to look on them as mere parts of the nervous system which governs them. It will be recollected that in treating of the skin, several terminal nervous organs have already been described (p. 68), to which the integuments owe their sensibility; and more complex organs are devoted to the special senses, which will hereafter be described. But at present we shall consider only the nervous centres and the nerves.

The nervous system, as developed in the invertebrate animals, consists of a series of ganglia connected in chains or other groups, and giving off nerves; but in the vertebrata it is divisible into two parts. One of these is the cerebrospinal system, consisting of the brain and spinal cord, together where it is exposed to the inhaled air, and when it reaches the arteries it is slightly colder than it was when in the right side of the heart, although it is not quite so cold as the blood in the jugular vein. That the blood should be cooled in passing through the lungs is contrary to all old beliefs, but it will not strike the student as strange when he considers how much heat is abstracted by the inhaled air before it quits the lungs. The absorption of oxygen by venous blood is proved experimentally to be accompanied with a certain evolution of heat; but the quantity is not sufficient to balance the loss by exposure to air inhaled at ordinary temperatures.

In disease, the temperature of the body may vary greatly from the healthy standard; in febrile affections it may rise to 106° or more, and in conditions of great feebleness, such as the collapse in cholera, it has been known to descend below 70°.

It will be understood, however, that the extremes of external temperature, which can be borne with impunity, are not accompanied with any such changes within the body, but illustrate the power which the body has of maintaining its own proper temperature. Thus, in extreme cold, the greater combustion necessary in the tissues is testified by the more active respiration; while in exposure to heat, the body is kept cool by evaporation. Temperatures far above what would be sufficient to boil the juices of the body, were they exposed directly to the heat, can be borne for a short time with impunity, provided always that the air be dry, so as to aid free evaporation from the surface; but moist air cannot be endured above a very moderate heat.