EFFICIENCY OF ISOLATED MUSCLE

397

As regards the work which has been carried out on isolated muscle, the results which have been obtained are of great interest, as they have led to fresh consideration of the nature of the muscular machine. A. V. Hill, in a long series of ingenious and striking experiments, using special methods of his own devising, has shown that the solution of the problem is not quite so simple as it was formerly imagined. Hill found the simple determination of the mechanical efficiency, i.e. W/H, the heat equivalent of the work done, divided by the energy output determined as heat, was of no real importance. The true efficiency of the muscle is the ratio between the "potential energy thrown into an active muscle by excitation" and the "total chemical energy liberated as heat." He found further, that the heat production varied according to whether the muscle was, or was not, allowed to shorten on stimulation. If shortening were permitted the heat output might be 30 per cent. smaller than if the muscle was prevented from shortening. On examination of the potential energy developed by a stimulated muscle not allowed to shorten, it was found to be approximately 1/6 Tl, where T = the maximum tension and the length of the muscle. Hill maintains that the true mechanical efficiency can be determined by comparing this quantity with the heat production. This value 1/6 T when expressed in heat units is 10-4/4.26 calories. (See Table LXXX.) He found efficiencies approximating 90 per cent. in the initial phases of contraction, and if the whole process, i.e. initial and recovery phases taken together, were assessed, the efficiency, under the conditions of his experiments, was in round figures 50 per cent.

TABLE LXXX.

EXPT. Length of muscles, 3-3 cm.; weight of muscles, 0-135 gm.; 1 scale division of deflection = 8.32 × 10-6 cal. Sartorius and isometric

Incidentally he found that different types of muscle (e.g. semimembranosous and sartorius) definitely differed in efficiency. He also found that the maximum efficiency was only obtained under very special conditions of initial tension, strength of stimulus and the physiological state of the muscle.

PART II

ILLUSTRATIVE EXPERIMENTS

1. Gaseous Diffusion. Experiment on page 40. Try this first with coal-gas and then with CO2. Soak the porous pot in water and compare the rate of diffusion inwards of carbon-dioxide with the outwards diffusion of air. What part does solubility play in diffusion through a

membrane ?

2. Osmotic Pressure of Crystalloids. Preparation of a Semipermeable Membrane. Take a clean porous pot such as is sold for Leclanché units. Allow it to soak for a day in distilled water. Fill it with a 0-25 per cent. solution of copper sulphate and immerse it in a 0.21 per cent. solution of potassium ferrocyanide for a day or two. Wash thoroughly in distilled water. The copper sulphate and potassium ferrocyanide meet in the porous pot and a membrane of copper ferrocyanide is there formed (see Expt. 6). The prepared pot may keep for years and be used many times.

A rubber stopper with two holes should be permanently fixed in its mouth with wax. Through one hole should be passed a long glass tube or a U-shaped glass manometer. The other hole carries a tap funnel for filling the pot. The solution to be tested should be coloured with methylene blue or other dye which is easily seen.

(1) What happens after 24 hours or so when a sugar solution is placed in the pot and the pot immersed in water?

(2) Now add sugar to the fluid outside the pot till its concentration is the same as that inside the pot and leave for the same period as before. (3) Increase the concentration of sugar outside and note the effect on the level of fluid in the manometer.

3. Blood Corpuscles. (1) Take three test tubes and place in one about 5 c.c. of water and in another a similar amount of 0.9 per cent. sodium chloride, and in the third 2 per cent. sodium chloride. Prick the finger with a sterile needle and add the same number of drops of blood to each tube. Shake and examine the tubes (a) as to opacity and (b) as to depth of colour. Take a drop of the fluid from each and examine under the microscope. Measure the diameter of a number of corpuscles and average those from each tube.

(2) Add a drop of fresh blood to a drop of 0.5 per cent. sodium chloride solution on a microscope slide. Place a card on the side of the microscope stage and keeping both eyes open trace the projection of a corpuscle from time to time or measure the diameter.

(See also Haematocrite, Expt. 47.)

OSMOTIC PRESSURE

399

4. Turgor (see Expt. 19, ii. for precautions). Take a length of sausage skin parchment. Close one end tightly round a glass stopper. Fill with treacle or a strong solution of sugar and then similarly close the other end. Suspend horizontally in water from a loop round the middle. The ends, which droop at first, giving the whole the appearance of an arch, soon begin to assume a horizontal position. In a day or so the sausage skin will be rigid and straight.

5. Chemical Gardens. (1) Place 50 c.c. of potassium ferrocyanide in a glass jar or beaker and add a small particle of ferric chloride (small pea). A semipermeable membrance of ferric ferrocyanide (Prussian blue) is formed round the solid. Endosmosis occurs and peculiar growths may be formed.

(2) Add a drop of almost saturated potassium ferrocyanide from the end of a glass rod to a solution of copper sulphate (bench reagent). A semipermeable membrane of copper ferrocyanide is formed round the drop and endosmosis takes place. This causes an increase in the concentration of the copper sulphate immediately round the drop, and blue rootlets may be seen descending from the drop. These are due to the increased density of the sulphate.

66

(3) Leduc's Growths. A small flat-sided jar, e.g. a specimen jar, is filled with a 1-2 per cent. solution of gelatine to which is added just enough potassium ferrocyanide to give it a pale green colour. Just before the gelatine has set, a little seed made from a mixture of glucose and copper sulphate is planted on the bottom of the jar. Within an hour, growth will be visible and may proceed for several days.

See Leduc, Études de Biophysique. I. Théorie Physico-Chimique de la Vie (1910); II. La Biologie Synthétique (1912).

6. Electric Endosmose. (a) The passage of water through a membrane by electrical means may be observed in the preparation of a semipermeable copper ferrocyanide membrane when the solutions are forced into the pores of the earthenware pot by an electrical current (Expt. 2).

(b) A clean porous pot, fitted with a manometer and a non-polarisable electrode, is filled with and placed in a solution of K2SO4 (0-05 per cent.). A current of 2-4 volts is passed so that the electrode inside the pot is cathode. Note the rise in level of the fluid inside the pot. Note also the increase in the alkalinity of the fluid outside the pot.

(c) Make a collodion test tube to fit one limb of the U-tube (Fig. 9).

(1) Fill both limbs with dilute K2SO4 solution. Mark the level of the fluid in both limbs and, using non-polarisable electrodes, pass a current of 4 volts for some time through the solution. Note that water passes towards the cathode and that the cathodal fluid becomes acid.

(2) Repeat, using tartaric acid in the collodion sac and pure water outside. Test for tartaric acid.

(3) Fill the sac with gelatine sol and leave overnight. Wash out the sol and repeat the expts.

7. Determination of the Freezing Point of Urine. Principle. The freezing point of water is depressed by the addition of salts which go into true solution. The magnitude of the depression termed A bears a relation to the molecular concentration of the solutes and therefore to their osmotic pressure.

Apparatus. Beckmann's (Fig. 74). It consists of a specially devised test tube 4 with a side neck. Through the rubber stopper, closing the main neck of this, pass a thermometer D and a short glass tubular

FIG. 74.-Freezing Point
Apparatus.

guide for a stirrer. The freezing point tube is supported in the neck of a large test tube B, by means of a cork or asbestos ring so that the freezing-point tube is protected from incoming heat by a mantle of still air. This ensures that the cooling of the liquid in the freezing-point tube is slow and fairly uniform. The whole apparatus is inserted through a hole in the middle of a brass sheet, to which it is fixed by a ring of cork or of asbestos. The sheet of brass acts as a lid to a glass jar C which contains powdered ice and salt-the cooling bath. Other holes in the lid permit of the passage of a stirrer, a thermometer, and a test tube containing pure water.

The thermo

The Beckmann Thermometer. meter in the freezing point tube must be graduated to, at least, hundredths of a degree. Such a thermometer, if made in the ordinary way, unless it were inconveniently long, would have a very short range. To obviate the necessity of having a series of thermometers for use over various ranges of temperature, Beckmann designed one which may be set to indicate temperatures over any desired range. This result is produced by a device permitting of alterations being made in the amount of mercury in the bulb. At the upper end of the thermometer there is a small reservoir into which the excess of mercury may be driven, or from which a larger supply of mercury may be

obtained.

Setting the Beckmann Thermometer. Hang the thermometer in a beaker of water, the temperature of which is 2-3 degrees higher than the highest temperature to be met with in the experiment and see whether or not the top of the mercury comes within the scale.

A. If there is too much mercury in the bulb and the column rises beyond the graduated part, the excess is removed by warming the mercury in the bulb till the column of mercury unites with the mercury in the reservoir. This is done, (a) by placing the bulb in water just a little warmer than before. (b) When the mercury passes to the top of the capillary tube and forms a small drop there, the thermometer should be carefully inverted and tapped gently so as to cause the mercury in the reservoir to coalesce with the mercury in the top of the capillary. (c) The thermometer is returned to the upright position by a gentle steady movement and its upper end is struck a sharp tap against the palm of the hand, causing

DETERMINATION OF FREEZING-POINT

401

the excess of mercury to break off from the end of the capillary. The thermometer is again tested in the first bath.

B. If, on the other hand, the amount of mercury in the bulb is so small that the top of the column does not rise to the bottom of the scale, more mercury will have to be drawn from the reservoir. The procedure is similar to that outlined above, but at (c) the thermometer is replaced in the first bath before breaking the mercury column. That is, the mercury in the bulb is allowed to contract and draw in more mercury from the reservoir before the connection between column and reservoir is broken by tapping. These operations are repeated till the proper level of mercury has been attained. This is always tested by placing the thermometer in baths having temperatures equal to the highest and lowest to be encountered in the experiment, and noting that the top of the column of mercury remains on the scale.

Method. (1) Set up the apparatus completely so as to ensure all parts fitting properly. See that the stirrer in the inner tube is working smoothly and does not strike against the bulb of the thermometer.

(2) Remove the thermometer and stirrer from the tube. Clean and dry the latter.

(3) Pipette in 25 c.c. of urine.

(4) Set the Beckmann thermometer so that, at 0° C., the mercury stands not lower than the middle of the scale.

(5) Dry the thermometer and insert it along with the stirrer in the freezing-point tube, so that the bulb of the thermometer is completely immersed in the urine.

(6) Fill the outer cooling vessel with water, ice and salt. The freezing point of urine can now be determined.

(7) First make an approximate determination by placing the freezingpoint tube directly in the cooling bath so that a rapid fall of temperature

Occurs.

(8) As soon as the urine shows signs of freezing remove the tube from the freezing mixture, dry it quickly and place it in the air jacket in the cooling bath.

(9) Stir slowly and read the temperature when it becomes constant. (10) Withdraw the tube and melt the ice by warming with the hand, trying to avoid raising the temperature more than 1° C.

(11) Rapidly dry the tube and reinsert it in the air jacket and repeat the freezing process, stirring slowly all the time.

(12) When the temperature has fallen to from 0.2° to 0.5° below the approximate freezing point found in (9) stir more vigorously. This generally is sufficient to induce solidification to commence and the temperature will now begin to rise.

(13) If so, stir slowly and take readings of the temperature every few seconds-tapping the thermometer each time before reading. Note the highest temperature reached.

(14) Again melt and repeat the determination. At least three determinations of the freezing point should be made, the mean being taken. The deviations of the chosen readings from the mean should be less than 0.002° C.

(15) The depression of the freezing point or, in this case, the thermometric readings may be converted into osmotic pressure in metres of water by multiplying by the factor 122.7.

B. B.

26

26