The recent description of the rat's cremaster muscle by Majno, Palade & Schoefl (1961) suggested that this muscle might provide a suitable site for direct observation of skeletal muscle blood vessels in the living animal. This proved to be so and the method developed allows the study of the reactions of the vessels to various stimuli applied topically and intravenously. It is thought that the vessels are seen in a state that, for a time at least, is nearly normal for resting skeletal muscle. The only comparable preparations that I can trace are those of Zweifach & Metz (1955a, b) and Hyman & Paldino (1962), but their observations do not cover the same ground as those now reported.The observations are directed more to exploring the possibilities of the method than to solving particular problems. They show, however, that in general the vascular reactions, in terms of dilatation and constriction, are in keeping with those derived from blood flow studies in man and other animals. They also show that the dilatation resulting from the intravenous administration of adrenaline affects mainly the arteries, as with acetylcholine, rather than the minute vessels, as with histamine. METHODS Animals. A rat of about 100 g body weight is anaesthetized with urethane (150 mg) sometimes with pentobarbitone sodium (3-0 mg) injected intraperitoneally, and placed on its back on a cork mat with the hind legs widely splayed. Body temperature is regulated by an electric lamp placed above and moved nearer or further away as required. Temperatures are measured by a thermo-electric thermometer (copper-constantan junctions); body temperature from a polythene covered junction inserted into the colon about 5 cm beyond the anus, tail temperature (when required) by a junction attached near its tip with zinc oxide plaster; that of the fluid irrigating the exposed muscle by a junction attached to the tip of the irrigating tube (Text- fig. 2).Blood pressure is measured (when required) from the carotid artery through a polythene tube, 0-58 mm internal diameter (PE 50, Clay Adams Inc., New York) and about 4 cm long, joined by a 3 mm (about) internal diameter tube (PE 330) to a miniature membrane manometer like that described by Sherrington (1919). Ringer's solution with heparin (10 u./ml.) perfused through the manometer at a rate of 0 5-1 0 ml./hr prevents clotting in the tube tip.

At present the most satisfactory estimates of blood volume are obtained from summing simultaneous measurements ofplasma volume, made by a dye method, and cell volume made by a marked cell method (Reeve, 1948 METHODS GeneralThe best method of injecting exact quantities of solutions and withdrawing blood samples was found to be through a fine polythene catheter passed via an ear vein into the great veins near the heart. The technique of catheterization, which has many applications, is described in an appendix. The catheter was maintained patent by a very slow infusion of heparinized saline. Injected fluids were washed in with a little saline; the first portion of a blood sample containing the saline from the catheter was discarded.All animals were starved for 12-15 hr. before an experiment. A preliminary (blank) blood sample of 4 ml. was withdrawn. The suspension of marked cells and the T 1824 solution were then injected and washed in. About 8 min. later a 4 ml. blood sample was withdrawn, and after this at about * Work undertaken on behalf of the Medical Research Council.

THE following simple device has been found serviceable for estimating the systolic blood-pressure in the central artery of the ear in the normal and unanaesthetized rabbit. It is based on von Recklinghausen's method [1906] and seems to us more satisfactory than any other we know; references to similar devices will be found in the papers by Kottlors and Rothschild [1932] and by Rai . When proper precautions are observed, the systolic pressure measured in the ear artery can be taken as a reliable index of systemic arterial pressure.

In the course of observations on the blood vessels of skeletal muscle in the rat, need arose for an index of the calibre of skin vessels in relation to body temperature. In the rabbit ear skin temperature is a useful index to the calibre of the ear vessels, which are largely controlled by body temperature (Grant, 1935). In the rat, however, observation showed that ear skin temperature did not seem to respond to changes of body temperature as it does in the rabbit. The matter was investigated further. Among other things, it was found that the vessels of the rat's ear do not dilate when the body is warmed and that the ear lacks arterio-venous anastomoses. METHODSAnimale. Rats and guinea-pigs were anaesthetized with pentobarbitone sodium, 1-5-3-0 mg/ 100 g body weight, injected intraperitoneally. The animals were warmed with an electric pad.Temperature was measured by thermo-electric thermometer, the junctions being made of fine (36 S.W.G.) copper and constantan wire. Body temperature was recorded from a junction covered in polythene and inserted into the colon 6 cm beyond the anus. To record skin temperature the hair was clipped short and the skin cleaned with ether; junctions were attached by adhesive zinc oxide plaster to the ear, feet and the rat's tail. These parts were shielded from the heat of the electric pad.The blood vessel8 were examined during life in the transilluminated ear by the naked eye and with a stereoscopic microscope (heat-filtered light). For microscopic examination ( x 12-5 and x 25 diam.), the depilated ear was spread on a Perspex sheet and covered with liquid paraffin. Change of vessel calibre was judged subjectively; in some instances, photomicrographs were taken.Sympathectomy. In the neck the sympathetic trunk was removed from the root of the neck up to and including the superior cervical ganglion. In the rat's abdomen both sympathetic chains were removed from below the coeliac ganglion to as far as they could be followed between the caudal muscles.Histological preparation8 were made by various methods suitable for detecting arteriovenous anastomoses. In the freshly killed animal blood was washed from the ear vessels by NaCl solution (0-9 g/100 ml. water), injected through cannulae inserted into the carotid arteries (under water, to avoid air embolism). The vessels were fixed in the dilated state with 10 % formalin injected under a pressure of 100-200 mm Hg for J-1 hr and then washed out with water. The vessels were then filled with Indian ink or stained with haematoxylin injected under pressure (Grant, 1929-31). The ear was separated into its various layers and these

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