examination of the chest, we shall not occupy much

The air-holder of Hutchinson is now familiarly known to the profession, and that of Pereira (which was devised after the apparatus arranged by Sir H. Davy) is of a similar nature, and both have the same

merits and defects. They are arranged to measure expiration, and are graduated to 400 or 500 cubic inches, which is more than enough for the largest expiration. They are so counterpoised as rather to favour the ascent of the inverted holder as it rises from the water, but there is no sufficient attempt to exactly balance the weight of the holder at different elevations. Hence, whilst they are sufficiently accurate for the purpose now under consideration, they cannot be regarded as instruments fitted for exact scientific research.

A modification of this spirometer has been made by M. Schnepf, so that by varying the weight of the chain in its different parts, it becomes a travelling counterpoise; and the weight of the holder is so exactly balanced in every part of its course, that stop-cocks and corrections become unnecessary. We insert a drawing of this, the least imperfect of this kind of spirometer. (See preceding page.)

Dr. Lewis, of Caermarthen, has recently introduced a very simple and cheap instrument, which fulfils the required indications with a sufficient degree of accuracy. The principle of its construction is the displacement of a volume of water equal to that of the air expired. The apparatus consists of a large glass jar, placed on its side at an angle, so as to favour the removal of the fluid, and furnished with an index, large tubes, and mouthpiece. When the graduation of such an instrument is carefully made, it becomes so far a perfect instrument, and the only defect to which we need to refer is the resistance which the mass of water

offers to the expiratory force; and although this is very appreciable at the end of expiration, when the force is greatly reduced, we are of opinion, after careful trial, that its indications are as reliable as those of much more expensive instruments. The following drawing has been kindly furnished by Dr. Lewis:

The spirometer which we have used in our various inquiries, is an inverted dry gas meter of improved construction, which has been adapted to measure the air in inspiration, and to register from one to one million cubic inches. It will, therefore, indicate the quantity of air inhaled at one full inspiration, as required in the determination of the vital capacity, or it will register the number of cubic inches inspired during ordinary respiration in an hour, or indeed for any

period however long or short. It is manufactured by Messrs. Croll, Rait and Co., Kingsland Road, N.E.

For the further discussion of this subject, we must refer to a series of papers on the Spirometer published by us in the "Medical Circular" of 1857.

CHAPTER XII.

INNERVATION.

Innervation is commonly lessened.

We are not able to enter at any length into the question of innervation, since the subject is at present too much involved in mystery to enable us to do more than indicate the general principles involved in that act.

There can be no doubt that, whilst there may be certain powers inherent in tissues, as, for example, in the muscular and fibrous tissues, there is a separate controlling influence, which not only calls forth the full power of action of each tissue and organ, but coordinates movements, and enables the body to act in its various parts simultaneously and as a harmonious whole. This, whatever it may be, acts through the nervous system, and hence a primary and controlling power is given to that system.

The amount of power which may thus be exerted varies with different conditions of the system, so that at one time the nervous force appears to be efficient, and the responses of the tissues which it calls into action are ready and effectual, whilst in others one or both of these conditions of force are weakened, and the resulting action is below the standard of health, without