There has been much discussion over the meaning of the rhythmic contractions observed in certain blood-vessels apparently independent of the central nervous system.' The median artery of the rabbit's ear, the arteria saphena in the same animal, and the vessels in the frog's web and frog's mesentery, slowly contract and relax. This rhythmic contraction is easily seen in the ear of a white rabbit. The movements are possibly of purely muscular origin, but are more probably the result of periodical discharges by vaso-motor nerve-cells.

Rhythmical variations in the tonus of the vaso-constrictor centres are often held to explain the oscillations seen in the blood-pressure curve after the influence of thoracic aspiration has been eliminated by opening the chest and cutting the vagus nerves. These oscillations are of two sorts. In the one, the blood-pressure sinks with every inspiration and rises with every expiration, though the rise and fall are not precisely synchronous with the respiratory movements; in the other, the so-called Traube-Hering waves, the oscillations embrace several respirations. It has also been suggested that these phenomena are due to periodical changes in the respiratory centre affecting the vaso-constrictor centre by "irradiation."

Vaso-motor Reflexes.-The vaso-motor nerves can be excited reflexly by afferent impulses conveyed either from the blood-vessels themselves or from the end-organs of sensory nerves in general. The existence of reflexes from the blood-vessels may be shown by Heger's experiment. Heger observed a rise of general blood-pressure with a subsequent fall, and at times a primary fall, after the injection of nitrate of silver into the peripheral end of the crural artery of a rabbit. The limb, with the exception of the sciatic nerve, was severed from the trunk. The quantity injected was so small that it probably was decomposed before passing the capillaries or escaping from the bloodvessels. Thus the effect exerted by the nitrate of silver on the general bloodpressure was probably caused by afferent impulses set up in the blood-vessels themselves and transmitted through the sciatic nerve to the vaso-motor centres. Vaso-motor reflexes are, however, much more commonly produced by the stimulation of sensory nerves other than those present in the bloodvessels.

The reflex constriction or dilatation 2 appears usually in the vascular area from which the afferent impulses arise. For example, the stimulation of the central end of the posterior auricular nerve in the rabbit causes a passing constriction followed by dilatation, or a primary dilatation often followed by constriction of the vessels in the ear. The stimulation of the nervi erigentes causes dilatation of the vessels of the penis. Gaskell found that the vessels of the mylo-hyoid muscle widened on stimulating the mucous membrane at the entrance of the glottis.

1 Franck: Archives de Physiologie, 1893, p. 729; Lui: Archives italiennes de Biologie, 1894, xxi.

p. 416; Goltz and Ewald: Archiv für die gesammte Physiologie, 1896, lxiii. p. 396.

2

Hegglin: Zeitschrift für klinische Medicin, 1894, xxvi. p. 25.

1

The vascular reflex may appear in a part associated in function with the sensory surface stimulated. Thus the stimulation of the tongue causes dilatation of the blood-vessels in the submaxillary gland. Frequently the vascular reflex is seen on both sides of the body. The stimulation of the mucous membrane on one side of the nose may cause vascular dilatation in the whole head; the effect in this case is usually more marked on the side stimulated. The vessels of one hand contract when the other hand is put in cold water. Sometimes distant and apparently unrelated parts are affected. Vulpian noticed that the stimulation of the central end of the sciatic caused the vessels of the tongue to contract.2

The vascular changes produced reflexly in the splanchnic area are of especial importance because of the great number of vessels innervated through these nerves and the great changes in the blood-pressure that can follow dilatation or constriction on so large a scale.

There is in some degree an inverse relation between the vessels of the skin and deeper parts on reflex stimulation of the vaso-motor centres. The superficial vessels are often dilated while those of deeper parts are constricted.3 Thus the stimulation of the central end of the sciatic nerve may cause a dilatation of the vessels of the lips, hand in hand with a rise in general blood-pressure. Exposing a loop of intestine dilates the intestinal vessels in the rabbit, but constricts those of the ear. In asphyxia, the superficial vessels of the ear, face, and extremities dilate, while the vessels of the intestine, spleen, kidneys and uterus are constricted.

Relation of Cerebrum to Vaso-motor Centres.-A rise of general bloodpressure has been produced by the stimulation of different regions of the cortex and of various other parts of the brain; for example, the crura cerebri and corpora quadrigemina. Vaso-dilatation has also been observed. The motor area of the cortex especially seems closely connected with the bulbar vasomotor centres. There is, however, no conclusive evidence that special vasomotor centres exist in the brain aside from the bulbar centres already described. At present the safer view is that the changes in blood-pressure called forth by the stimulation of various parts of the brain are reflex actions, the afferent impulse starting in the brain as it might in any other tissue peripheral to the

vaso-motor centres.

Pressor and Depressor Fibres.-The stimulation of the same afferent nerve sometimes causes reflex dilation of the vessels of a part, instead of the more usual reflex constriction. Two explanations of this fact have been suggested. The first assumes that the condition of the vaso-motor centre varies in such a way that the same stimuli might produce contrary effects, depending on the relation between the time of stimulation and the condition of the centre. 'The general arrangement of the matter in this paragraph is that given by Tigerstedt, Der Kreislauf, 1893, p. 519.

Compare Sergejew: Centralblatt für die medicinische Wissenschaft, 1894, p. 162. Wertheimer: Comptes rendus, 1893, cxvi. p. 595; Hallion and Franck: Archives de Physiologie, 1896, p. 502; Bayliss and Bradford: Journal of Physiology, 1894, xvi. p. 17. 4 Isergin: Archiv für Physiologie, 1894, p. 448.

The second assumes the existence of special reflex constrictor or "pressor fibres, and reflex dilator or "depressor" fibres. The existence of at least one depressor nerve is beyond question, namely the cardiac depressor nerve, which it will be remembered runs from the heart to the bulb and when stimulated causes a dilatation of the splanchnic and other vessels reflexly through the bulbar vaso-motor centre. Evidence of other reflex vaso-dilator nerves and of reflex vaso-constrictor fibres as well has been offered by Latschenberger and Deahna, Howell,' and others. Howell, for example, has found that if a part of the sciatic nerve is cooled to near 0° C. and the central end stimulated peripherally to this part, the blood-pressure falls, instead of rising, as it does when the nerve is stimulated without previous cooling. Howell's experiments have been recently extended by Hunt, who finds that the stimulation of the sciatic during its regeneration after section gives at first vaso-dilatation only, but when regeneration has progressed still further, vaso-constriction is secured. These results point to the existence of both pressor and depressor fibres, the latter being the first to regenerate after section. A reflex fall in blood-pressure is also produced by stimulating various mixed nerves with weak currents and by the mechanical stimulation of the nerve-endings in muscle. The fall is more readily obtained when the animal is under ether, chloroform, or chloral, less readily under curare.

Topography. We pass now to the vaso-motor nerves of various regions. Brain. The study of the innervation of the intracranial vessels is rendered exceptionally difficult by the fact that the brain and its blood-vessels are placed in a closed cavity surrounded by walls of unyielding bone. The fundamental difference created by this arrangement between the vascular phenomena of the brain and those of other organs was recognized in part at least by the younger Monro as long ago as 1783. Monro declared that the quantity of blood within the cranium is almost invariable, "for, being enclosed in a case of bone, the blood must be continually flowing out of the veins that room may be given to the blood which is entering by the arteries, as the substance of the brain, like that of the other solids of our body, is nearly incompressible." Further differences between the circulation in the brain and in other organs are introduced by the presence of the cerebro-spinal fluid in the ventricles and in the arachnoidal spaces at the base of the brain. This fluid may pass out into the spinal canal and thus leave room for an increase in the amount of blood in the cranium. Finally, a rise of pressure in the arteries too great to be compensated by the outflow of cerebro-spinal fluid may lead to compression of the venous sinuses and a decided change in the relative distribution of the blood in the arteries, capillaries and veins-conditions which are not present in extracranial tissues. It is evident, therefore, that the methods employed in the search for vaso-motor nerves within the cranium must take

'Howell, Budgett, and Leonard. Journal of Physiology, 1894, xvi. p. 310; Bayliss: Ibid., 1893, xiv. p. 317; Bradford and Dean: Ibid., 1894, xvi. p. 67; Hunt: Ibid., 1895, xviii. p. 381. Cavazzani: Archives italiennes de Biologie, 1893, xviii. p. 54, xix. p. 214; Bayliss and Hill: Journal of Physiology, 1895, xviii. p. 334; Gulland: Ibid., p. 361.

into account many sources of error that are absent in vaso-motor studies of other regions. It is, indeed, probable that incompleteness of method will go far toward explaining the disagreement of authors as to the presence of vasomotor nerves in the brain. According to Bayliss and Hill, who have recently studied this subject, it is necessary to record simultaneously the arterial pressure, the general venous pressure, the intracranial pressure and the cerebral venous pressure, the cranium as in the normal condition being kept a closed cavity. In their experiments, "a cannula was placed in the central end of the carotid artery. A second long cannula was passed down the external jugular vein, and on the same side, into the right auricle. The torcular Herophili was trephined, and a third cannula, this time of brass, was screwed into the hole thus made." The intracranial pressure was recorded by a cannula connected through another trephine-hole with the subdural space.

Bayliss and Hill could find no evidence of the existence of cerebral vasomotor nerves. The cerebral circulation, according to them, passively follows the changes in the general arterial and venous pressure. Gulland has examined the cerebral vessels by the Golgi, Ehrlich, and other methods, to determine whether nerve-fibres could be demonstrated in them. None were found. It is probable that the blood-supply to the brain is regulated through the bulbar vaso-constrictor centre. Anæmia or asphyxia of the brain stimulates the cells composing this centre, vascular constriction of many vessels follows, and more blood enters the cranial cavity. The vessels of the splanchnic area play a chief part in this regulative process.1 Their importance to the circulation in the brain is shown by the fatal effect of the section of the splanchnic nerves in the rabbit. On placing the animal on its feet, so much blood flows into the relaxed abdominal vessels that death may follow from anæmia of the 'brain.

Vaso-motor Nerves of Head.-The cervical sympathetic contains vaso-constrictor fibres for the corresponding side of the face, the eye, ear, salivary glands and tongue, and possibly the brain. The spinal vaso-constrictor fibres for the vessels of the head in the cat and dog leave the cord in the first five thoracic nerves; in the rabbit, in the second to eighth thoracic, seven in all.

Vaso-dilator fibres for the face and mouth have been found in the cervical sympathetic by Dastre and Morat, leaving the cord in the second to fifth dorsal nerves, and uniting (at least for the most part) with the trigeminus by passing, according to Morat, from the superior cervical sympathetic ganglion to the ganglion of Gasser. Other dilator fibres for the skin and mucous membrane of the face and mouth arise apparently in the trigeminus, for the stimulation of this nerve between the brain and Gasser's ganglion causes dilatation of the vessels of the face, and in the nerve of Wrisberg.

The vaso-motor nerves of the tongue have been recently studied by Isergin.3

1 Wertheimer: Archives de Physiologie, 1893, p. 297.

Langley: Philosophical Transactions, 1892, p. 104; Piotrowsky: Centralblatt für Physiologie. 1892, vi. p. 464. Isergin: Archiv für Physiologie, 1894, p. 441.

3

The lingual and the glosso-pharyngeal nerves are recognized by all authors as dilators of the lingual vessels. The sympathetic and the hypoglossus contain constrictor fibres for the tongue. It is possible that the lingual contains also a small number of constrictor fibres. Most if not all these vaso-motor fibres arise in the sympathetic and reach the above-mentioned nerves by way of the superior cervical ganglion. They degenerate in from three to five weeks after the extirpation of the ganglion.

Morat and Doyon cut the cervical sympathetic in a curarized rabbit and examined the retinal arteries with the ophthalmoscope. They were found dilated. The excitation of the cervical sympathetic caused constriction, the excitation of the thoracic sympathetic dilatation of these vessels. The retinal fibres leave the sympathetic at the superior cervical ganglion and pass along the communicating ramus to the ganglion of Gasser, whence they reach the eye through the ophthalmic branch of the fifth nerve, the gray root of the ophthalmic ganglion, and the ciliary nerves. Most, or all, of the fibres for the anterior part of the eye are found in the fifth nerve.

Lungs.-The methods ordinarily employed for the demonstration of vasomotor nerves cannot without danger be used in the study of the innervation

FIG. 45.-The excitation of the central end of the inguinal branch of the crural (sciatic) nerve causes a rise in the aortic pressure (Pr. A. F.), a rise in the pressure in the pulmonary artery (Pr.A. P.) of 10 to 16 mm. Hg, accompanied by a falling pressure in the left auricle (Pr.O.G.) (Franck, 1896, p. 184). The rise of pressure in the pulmonary artery, together with the fall in the left auricle, demonstrate, according to Franck, a constriction of the pulmonary vessels.

of the pulmonary vessels. A fall in the blood-pressure in the pulmonary artery, for example, produced by stimulating any nerve cannot be taken as final evidence that the stimulation caused the constriction of the pulmonary vessels. The lesser circulation is so connected that changes in the calibre of the vessels of a distant part, the liver for example, may alter the quantity of blood in the lungs. The method of Cavazzani avoids these difficulties. Cavazzani establishes an artificial circulation through one lobe of a lung in

1Doyon: Archives de Physiologie, 1893, p. 101; Henriques: Skandinavisches Archiv für Physi ologie, 1893, iv. p. 229; Bradford and Dean: Journal of Physiology, 1894, xvi. p. 34; Franck: Archives de Physiologie, 1896, p. 178.