Page:EB1911 - Volume 27.djvu/963

Rh likewise have their centre in the spinal bulb and are in tonic action, antagonizing more or less the action of the vagal centre.

From Howell’s Text-Book of Physiology, by permission of W. B. Saunders Co.

. 18.—B, arterial blood pressure. K, record of volume of kidney. Inhibition of heart on faradizing vagus nerve.

The vagus nerve works directly on the cardiac muscle, and produces some change (signalized by a positive variation in the electrical state of the heart) which results in a depression of the excitability, the conductivity, the force and the frequency of the heart. After the vagal arrest the heart beats more forcibly, owing, it is thought, to the greater accumulation of contractile material during the period of rest. The converse of all these effects occurs on stimulation of the accelerator nerves. Excitation of these nerves may excite to renewed efforts an excised heart which has just ceased to 2 beat after withdrawal of the supply of nutritive solution. Hence it is thought by some, that the accelerator nerves tonic ally exert a sustaining influence on the heart.

The alkaloid atropin paralyses the vagal nerve endings in the heart, while nicotine paralyses the ganglion cells. Muscarin obtained from poisonous fungi slows and finally arrests the heart. Adrenalin, the active principle of the medulla of the supra-renal glands, augments its power. Chloroform depresses it and in poisonous dose throws the heart into paralytic dilatation. A great many of the cardiac vagal fibres convey impulse to the spinal bulb (centripetal), and redexly influence the heart frequency, the breathing and the tonus of the blood vessels. In particular certain fibres, termed depressor (discovered by Ludwig and Cyon, 1866), cause dilatation of the arterioles and a fall of arterial pressure by inhibiting the tonic action of the vaso-motor centre in the spinal bulb. The depressor fibres arise from the root of the aorta, and over distension of this part excites them, as evidenced not Only by the above effect, but also by the electrical variation (action current) which has been observed passing up the depressor nerve. Sensory impressions originating in the heart do not as a rule enter into consciousness. They are carried by the cardiac nerves to the sympathetic ganglia, and thence to the upper thoracic region of the spinal cord, where they come into relation with the sensory nerves from the pectoral region, upper limb, shoulder, neck and head. The impressions are not felt in the heart, but referred to these sensory cutaneous nerves. Thus cardiac pain is felt in the chest wall and upper limbs and particularly on the left side. The function of the cardiac nerves is to co-ordinate the beat of the heart with the needs of the body and to co-ordinate the functions of other organs with the needs of the heart. For example, an undue rise of arterial pressure, induced, let us say, by compression of the abdomen, excites the centre of the vagus and produces slowing of the heart and a consequent lowering of arterial pressure. The heart of a mammal, however, continues to functionate after a section of all the branches of the cardiac plexus has been made, so that the nervous control and co-ordination of the heart are not absolutely essential to the continuance of life.

Water flowing through a tube from a constant head of pressure encounters a resistance occasioned by the friction of the moving water particles against each other and against the stationary layer that wets the wall of the tube. Part of the potential energy of the head of pressure is spent in endowing the fluid with kinetic energy, the greater part in overcoming this resistance is rubbed down into heat. The narrower the tube is made, the greater the friction, until finally the flow ceases, the total energy being then insufficient to overcome the resistance.

The resistance may be measured at any point in the tube by inserting a side tube in the vertical position. The water rises to a certain height in the side tube, indicating the head of pressure spent in overcoming the resistance between the point of measurement and the orifice. If the lower end of the side tube is bent thus ⅃ and inserted so that its orifice faces the stream, the water will rise higher than it did in the first case. The extra rise indicates the head of pressure spent in maintaining the velocity of flow. Such a method has been used to measure the velocity of flow in the vascular system (Napoleon Cybulski). When a stream of water is transmitted intermittently by the frequent strokes of a pump through a long elastic rubber tube, the fluid does not issue in jets as it would in the case of a rigid tube, but flows out continuously. The elastic tube is distended by the force of the pump, and its elasticity maintains the outflow between the strokes. The continuous outflow here depends on the elasticity of the tube and the resistance to flow.

In the vascular system an area of vessels of capillary size is placed between the large arteries and veins. This area opposes a great resistance to flown The arteries also are ex tensile elastic tubes. The effect of the peripheral resistance, as it is called, is to raise the pressure on the arterial side and lower it on the venous. The resistance to flow is situated chiefly, not in the capillaries, but in the small arteries, where the velocity is high; for “skin friction”that is, the friction of the moving concentric layers of blood against one another and against the layer: which wets the wall of these blood vessels is proportional to the surface area and to the viscosity of the blood—is nearly proportional to the square of the velocity of flow, and is inversely proportional to the sectional area of the vessels. Owing to the resistance to the capillary outflow, the large arteries are expanded by each systolic output of the heart, and the elasticity of their walls comes into play, causing the outflow to continue during the succeeding diastole of the heart. The conditions are such that the intermittent flow from the heart is converted into a continuous flow through the capillaries. If the arteries were rigid tubes, it would be necessary for the heart to force on the whole column of blood at one and the same time; but, owing to the elasticity of these vessels, the heart is saved from such a prolonged and jarring strain, and can pass into diastolic rest, leaving the elasticity of the distended arteries to maintain the flow. As a result of disease, the elastic tissue may degenerate and the arteries become rigid., Besides the saving of heart-strain, there are other advantages in the elasticity of the arteries. It has been found that an intermittently acting pump maintains a greater outflow through an elastic than through a rigid tube; that is to say, if the tubes be of equal bore; The four chief factors which co-operate in producing the conditions of pressure and velocity in the vascular system are—(1) the heart-beat, (2) the peripheral resistance, (3) the elasticity of the arteries, (4) the quantity of blood in the system. Suppose the body to be in the horizontal position and the vascular system to be brought to rest by, say, excitation of the vagus nerve and arrest of the heart. A sufficiency of blood to distend it collects within the venous cistern. The arterial system, owing to its elasticity and contractility, empties. If the heart now begin to beat, blood is taken from the venous system and is driven into the arterial system., The arteries receive more blood than can escape through the capillary vessels, and the arterial side of the system becomes distended, until equilibrium is reached, and as much blood escapes into the venous side per unit of time as is delivered by the heart. The flow in the capillaries and veins has now become a constant one and if the side pressure be measured it will be found to fall from the arteries to the capillaries, and from the capillaries to the venae cavae. In the large arteries there is a large side pressure which rises and falls with the pulses of the heart. The pulse waves spread out over a wider and wider area as the arteries branch. They finally die away in the arterioles. An increase or decrease in the energy of the heart-beat will increase or decrease respectively the velocity of flow and pressure of the blood. An increase or decrease in the total width of the arterioles respectively will lessen or raise the resistance; increase or decrease the velocity; lower or raise the blood pressure. A loss of blood, other conditions remaining the same, would cause a decrease in pressure and velocity. As a matter of fact, such a loss is compensated for by the adjust ability of the vascular system. Tissue lymph passes from the tissues into the blood. and the blood vessels of the limbs and abdomen constrict, and thus the pressure is kept up, and an efficient circulation maintained through the brain, lungs and coronary vessels of the heart.

The whole vascular system is lined within by a layer of flattened cells, the endothelium; each cell is exceedingly thin and cemented to its fellows by a wavy border of an interstitial protoplasmic substance. The endothelium affords a smooth surface along which the blood can flow with ease. Outside it there exists in the arteries and veins a middle and an external coat. The middle coat varies greatly in thickness and contains most of the non-striated muscle-cells, which in the smaller arteries and arterioles form; a particularly well developed band. In the larger arteries (fig. 19) a great deal of yellow elastic tissue, together with some white, fibrous tissue, pervades the middle coat. At the inner and outer border of this coat the elastic fibres fuse to form an internal and external fenestrated membrane. This coat endows the arteries with extensibility, elasticity and contractility. The outside coat consists mostly of white fibrous tissue and not only protects the arteries, but by its rigidity prevents over-distension. In the veins (fig. 20), where the middle coat is somewhat thinner and contains less elastic tissue, the outer coat consists mostly of muscle-fibres. The valves of the veins are formed of fibrous and elastic tissue covered with endothelium. As the arterioles branch into capillaries the muscular and elastic elements become less and less, until in the capillaries themselves there is left only the layer of endothelium, supported by some stellate connective tissue cells The