62 THE MECHANISM OF THE CIRCULATION. 



After clamping the thoracic aorta, or dividing the spinal cord in the 

 dorsal region, excitation of the depressor still causes a fall of arterial 

 pressure. This fall is abolished by section of the cervical sympathetic 

 nerves. The vessels of the head and neck therefore share in the pro- 

 duction of the depressor effect, and, on plethysmographing the tongue, 

 this organ is found to dilate. So, too, the thigh muscles, free from skin, 

 and the foot, which is practically all skin and bone, are both found to dilate 

 (Bayliss). Thus we may conclude that the action of the depressor is almost 

 universal, nevertheless the splanchnic area is by far the most important 

 seat of dilatation. It is by means of this afferent nerve that the heart can 

 relieve itself of too great a systolic strain. The splanchnic floodgates can 

 be thrown rapidly open thereby, and the peripheral resistance lowered. 



When an animal is placed in the feet-down position during excitation 

 of the depressor, the arterial pressure, owing to splanchnic dilatation, 

 falls to a very marked extent. 1 



H^EMODYXAMICS. 



The circulation of the blood follows certain definite laws ; unfortunately 

 the conditions of flow are so complicated that these laws remain for the most 

 part still undetermined. 



A viscous fluid driven by an intermittent pump which circulates through a 

 system of branching elastic tubes of varying capacity ; a system of tubes into 

 and out of which passage of fluid takes place either by osmosis, filtration, or 

 secretion ; a fluid which varies in viscosity ; a pump which varies in force, and 

 tubes which have an ever-changing diameter and coefficient of elasticity. It is 

 in the first case helpful to consider the flow of fluids in certain simple models, 

 where the conditions are constant. The application of theoretical deductions 

 thus obtained must be tested by experimental inquiry on the living animal. 



Torricelli's law. If a circular hole be pierced in the bottom of a vessel 

 of water, the outflowing fluid, according to the law deduced by Torricelli, should 

 have a velocity F= v / 'Igh. 



Now, the velocity of a free falling body, when it has fallen through a space 

 1i, equals \J 2gh. Therefore the velocity of the fluid escaping through the 

 hole in the bottom of the vessel is equal to that which the fluid would attain 

 if it had fallen free through the height of the column of water. In other 

 words, the velocity of outflow is that velocity which a free falling body, starting 

 from a state of rest at the surface, would gain on reaching the orifice. Given 

 the velocity of outflow, and the sectional area of the orifice, the product of 

 these two is equal in a given time to the volume of the output. The output 

 is equivalent to the cylindrical column of fluid which has fallen through 

 the orifice in the given time. If A is the volume of the output, then 

 A = Trr 2 x / 2gh. 



Experiment, however, yields an actual result which shows that the outflow 

 is less than that theoretically deduced. This is because the motion at the 

 orifice is not steady. Eddies are there set up ; and in consequence of their 

 counter-action, the well-known phenomenon of the vena contracta is produced. 



Thus, whenever fluid in motion meets a sharp edge, there is a breach of 

 continuity and a consequent loss of energy. This loss can be avoided by 

 imitating the form of the vena contracta and making the orifice bell-shaped. 

 With a bell-shaped orifice the observed velocity is 0'99 times the theoretical 

 value. Such an arrangement exists in the branches of the aorta. Thoma has 

 determined this by injecting molten paraffin at blood pressure into the arterial 

 circulation, and cutting sections at the points of branching. The shape of 

 1 Hill and Barnard, Journ. Physiol., Cambridge and London, 1897, vol. xxi. p. 335. 



