298 THE CIRCULATION IN THE BLOOD-VESSELS [GIL XXI. 



the body in such a way that the body temperature shall remain 

 constant ; when excess of heat is produced there is also an increase 

 in the loss of heat ; the skin vessels are then dilated, and so more 

 blood is exposed on the surface, and thus an increase in the radiation 

 of heat from the surface is brought about. On the other hand, when 

 it is necessary that the heat produced should be kept in the body, 

 the loss of heat is diminished by a constriction of the skin vessels, 

 as in cold weather, The alteration of the calibre of the vessels is 

 brought about by the action of the vaso-motor nervous system on 

 the muscular tissue of the arterioles. 



There are certain organs of the body in which the necessity for 

 alterations in their blood-supply does not exist. Such organs are 

 the lungs and the brain. It is in the vessels of these organs that the 

 influence of vaso-motor nerves is at a minimum. The pulmonary 

 vessels are stated by Bradford and Dean to be supplied by nerves 

 which leave the cord in the upper thoracic region ; but on stimulating 

 these the rise of pressure produced is extremely small ; it is very 

 doubtful if the fibres in question are really vaso-constrictors ; the 

 small rise observed may be partly or even wholly due to the accelera- 

 tion of the heart, which is anothor result of stimulating these nerve- 

 roots. 



The vaso-motor centre lies in the grey matter of the floor of the 

 fourth ventricle ; it is a few millimetres in length, reaching from the 

 upper part of the floor to within about 4 mm. of the calamus scrip- 

 torius. The position of this centre has been discovered by the 

 following means : when it is destroyed the tone of the small vessels 

 is no longer kept up, and in consequence there is a great and universal 

 fall in arterial blood-pressure; when it is stimulated there is an 

 increase in the constriction of the arterioles all over the body, and 

 therefore a rise of arterial blood-pressure. Its upper and lower 

 limits have been accurately determined in the following way : a series 

 of animals is taken, and the central nervous system divided in a 

 different place in each; the cerebrum and cerebellum may be cut 

 off without affecting blood-pressure, the vaso-motor centre must 

 therefore be below these; if the section is made just above the 

 medulla, the blood-pressure still remains high, and it is not till the 

 upper limit of the centre is passed that the blood-pressure falls. 

 Similarly, in another series of animals, if the cervical cord is cut 

 through, and the animal kept alive by artificial respiration, there is 

 an enormous fall of pressure due to the influence of the centre being 

 removed from the vessels ; in other experiments the section is made 

 higher and higher, and the same result noted, until at last the lower 

 limit of the centre is passed, and the fall of pressure is less and less 

 marked the higher one goes there, until in the animal in which the 

 section is made at the upper boundary of the centre the blood- 



