VASOMOTOR NERVE FIBRES TO THE MAMMARY GLANDS OF THE CAT AND DOG

1. The existence of sympathetic vasoconstrictor fibres in the spinal nerves to the mammary glands of the cat and dog has been demonstrated in experiments involving blood flow measurements, plethysmography, perfusion of the isolated gland, and microscopical examination of the living vessels. 2. In the cat, vasoconstriction was only observed in the small arteries and arterioles, vessels which, on histological examination, were found to have much smooth muscle in the medial coat and were innervated by fine unmyelinated nerve fibres. 3. The nerve endings concerned are thought to be adrenergic, because— (a) adrenaline has the same action in very small doses; (b) their effects are unaffected by eserine, atropine and nicotine; (c) are potentiated by cocaine and tannic acid; (d) abolished or reversed by ergot preparations. 4. No convincing evidence has been obtained of the existence of active vasodilator fibres in the spinal nerves studied, or of parasympathetic innervation of the mammary gland. 5. The mammary blood vessels are very sensitive to cooling, to which they react by vasoconstriction. 6. Two types of spontaneous volume waves seen in plethysmograph experiments are described and their causes discussed.