THE BLOODVESSELS 85 



This return path to the spinal nerves is not followed by the 

 constrictor fibres which supply the bloodvessels of the head 

 and neck. These come off at first from the thoracic roots of 

 the spinal cord, and having passed through the vertebral sym- 

 pathetic ganglion, they proceed to the inferior cervical ganglion, 

 and by means of the cervical sympathetic gain the superior 

 cervical ganglion. From this the constrictor fibres for the head 

 issue. 



The constrictor fibres to the bloodvessels of the abdominal 

 viscera have another arrangement. They come out from the 

 spinal cord as the greater and lesser splanchnics, and pass to the 

 semilunar ganglion, in which the nerve enters and ends ; from 

 the ganglion more non-medullated nerves issue, and pass direct 

 to the vessels of the viscera. 



The essential feature in all these constrictor fibres is that they 

 originate in the brain or cord, leave the latter as medullated 

 nerves, and enter a sympathetic ganglion, where they terminate 

 by arborising around cells in the ganglion. From these cells 

 new fibres arise, which leave the ganglion as non-medullated 

 nerves, and proceed to their destination either direct, as in the 

 head, neck, and viscera, or reach it through the spinal nerves. 



If the spinal cord be divided below the medulla, and life main- 

 tained by means of artificial respiration, the immediate effect of 

 division is a great fall of blood-pressure, due to dilatation of the 

 bloodvessels ; in the dog it will drop two-thirds below the normal. 

 The effect of division has been to cut off the constrictor influence, 

 which was evidently issuing from some point above the section. 



If in another animal the section be made above the medulla, 

 no effect is produced on the blood-pressure. Evidently, there- 

 fore, the medulla contains a centre presiding over the important 

 functions of maintaining the bloodvessels in the partially con- 

 tracted condition known as tone, and it can readily be shown 

 that this centre lies in the region of the fourth ventricle, and is 

 only a few millimetres in length and still fewer in breadth. To 

 this small area the name vasomotor centre has been given. 



This centre, we have learnt, must be kept in a constant 

 state of activity. This is effected, though perhaps not entirely, 

 by the constant flow of impulses from the periphery to the centre. 

 Impulses passing to a centre from without to within are known 

 as afferent, those passing out from a centre to the periphery are 

 spoken of as efferent impulses, and a collection of nervous matter 

 where afferent impulses are received and efferent discharged is 

 known as a reflex centre. 



The afferent impulses which govern the centre in the medulla 

 are carried to it through the spinal nerves from areas such as 

 the skin and abdominal viscera, for these are the two great 



