Monary Circulation

It can easily be shown that the continuous stream of impulses descending to the blood vessels along the vasoconstrictor fibres originates in the central nervous system. If the spinal cord of a dog is divided below the medulla, the blood pressure falls from about 12o mm. of mercury to about so mm. Stimulation of the divided spinal cord immediately causes an increase in blood pres sure, which may be as high as 280 mm. or more. Section of the central nervous system above the medulla has no lasting effect on the blood pressure. As the result of these experiments we may say that the vessels of the body are kept in a state of in creased tone by impulses arising in the medulla. The portion of the medulla concerned with sending out these impulses is spoken of as the vasomotor centre. The activity of this centre is con stantly under the influence of various afferent stimuli originating within the various organs of the body or as the result of excita tions of higher or lower parts of the central nervous system.

The Vasoconstrictor and Vasodilator Fibres.

The vaso constrictor fibres leave the spinal cord in the thoracic region. They have a cell station in the ganglia of the sympathetic chain, and are then distributed along various nerves to the organs. The most important vasoconstrictor nerve is the splanchnic nerve, which supplies all the abdominal organs.

The typical vasodilator nerves take their origin from various parts of the central nervous system. Thus the vasodilators to the salivary glands and tongue are distributed with the cranial nerves, and probably originate from the nervus intermedius be tween the seventh and eighth cranial nerves. The nervi erigentes going to the genitalia leave the lower end of the cord by the anterior roots of the second and third sacral nerves. The vaso dilator fibres going to some of the abdominal organs and to the extremities have the same origin and course as the ordinary sensory nerves. Since the vasodilator impulses pass along these nerves in a direction opposite to that taken by the normal sensory impulses, they have been termed antidromic impulses. This appears to be one of the few cases in mammals in which the same nerve fibre performs two different functions.

The vasodilator and vasoconstrictor fibres are generally mixed in common trunks innervating an organ, and it is only by special means that they can be functionally separated.

Peripheral Tone of the Blood Vessels.

It has been men tioned above that the function of the vasomotor nerves is only to change the tonic condition of the blood vessels ; the tone itself does not disappear, however, even after complete denervation. This peripheral tone of the blood vessels is considerably influ enced by the composition of the blood, especially the oxygen tension, the degree of alkalinity, and the presence of vasocon strictor substances which are normally secreted by the pituitary and the suprarenal glands. Whether the nerves are intact or

severed, the capillaries of the body are never all open at the same time. The capillary bed seems to be, at least in some organs, in a perpetual state of variation. Capillaries constantly close and open so that blood flows through various channels. Whenever the oxygen tension of the blood diminishes, the blood vessels dilate, a large number of capillaries open, and the blood flow through the organ increases ; excess of carbon dioxide has the same effect.

On the other hand, an increase of the pituitary secretion or of adrenaline in the blood increases the tone of the blood vessels and capillaries. Some authors regard these two substances as be ing primarily responsible for the peripheral tone of the blood vessels in general. Superimposed upon these perpetual variations of the peripheral tone is the effect of the vasomotor nerves, and it often happens that agencies which would affect the peripheral tone in some definite direction are prevented from doing so on account of antagonistic influences from the vasomotor centre. For instance in the case of increased carbon dioxide tension of the blood, the carbon dioxide acting peripherally tends to dilate the blood vessels, but it is also a strong stimulus to the vasomotor centre and excites the vasoconstrictor fibres to such an extent that the blood vessels constrict ; the blood pressure then rises until the central nervous system succumbs to the toxic effect of the carbonic acid.

The Vasomotor Centre.

Besides being sensitive to carbon dioxide and to the acid products of metabolism which accumulate in consequence of lack of oxygen, the vasomotor centre is con stantly influenced by impulses arriving from the sense organs on the surface of the body and from various other organs. Amongst these, impulses originating in the vascular system are the most important, and are practically the only ones which have been studied. The afferent stimuli may be of two kinds. First are those which affect the vasomotor centre in such a way as to cause vasoconstriction either in some definite organ or in many organs ; in the latter case the general vascular resistance increases and the arterial pressure rises. These afferent impulses are described as pressor impulses. On the other hand, impulses may reach the vasomotor centre which will lead to a vasodilation ; this again may be restricted to a small area or may involve sufficient blood vessels to bring about a fall in the arterial blood pressure. These are known as depressor impulses.