412 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



direction. The former convey impulses from all parts of the body to 

 the center and the latter, from the cent-er to the blood-vessels (Fig. 

 221).. But as the lumen of the blood-vessels may be either decreased 

 or increased in size, the efferent or motor path must be composed of 

 two' types of fibers, namely, those which diminish and those which 

 enlarge it. The former are designated as vasoconstrictors and the 

 latter as vasodilators. In accordance with this functional division of 

 the fibers, it is possible to look upon the vasomotor center as being 

 composed of a vasoconstrictor and a vasodilator part. 



As the afferent impulses arriving in the center are capable of 

 producing either a vasoconstriction or a vasodilatation, the fibers 

 conducting them are commonly designated as pressor and depressor 

 fibers. Thus, if an impulse is generated either in the center or along 

 the course of an efferent nerve, and produces a constriction of the 

 blood-vessels, the reaction is spoken of as a vasoconstrictor action. 

 Again, if the stimulation of the same constituents of the vasomotor arc 

 leads to a dilatation of a certain area of blood-vessels, the effect is said 

 to be vasodilator in its nature. But, if the stimulus arises in a re- 

 ceptor or along the course of an afferent nerve, the reaction is desig- 

 nated as pressor if constrictory, and as depressor, if dilatory in its 

 nature. The last two terms, therefore, signify that the vascular reac- 

 tions have been brought about reflexly. 



The Location of the Vasomotor Center. — Nerve fibers, regulating 

 the caliber of the blood-vessels, may be contained in almost any 

 nerve, together with fibers possessing other functions. They may also 

 be grouped in such large numbers that they form individual nerve 

 strings of considerable size. But whether mixed with other fibers 

 or pursuing an independent course, they cannot be differentiated from 

 fibers possessing a different function excepting by physiologic means. 

 In other words, as nerve fibers bear no special points of difference in 

 their appearance, their function must be arrived at by subjecting them 

 to certain physiological procedures, such as mechanical and electrical 

 stimulation. 



It is a well-known fact that the division of the spinal cord in the 

 cervical region gives rise to an extensive relaxation of the blood- 

 vessels and a fall in the general blood pressure, while the division of 

 the nervous system above the upper border of the meduUa remains 

 without effect. From this it may be inferred that the separation of 

 the peripheral nerve paths from the brain occasions a loss in the tonus 

 of the blood-vessels ordinarily imparted to them by ganglion cells situ- 

 ated between these two cuts. Repeated experimentation has finally 

 led to the localization of a colony of cells in the medulla oblongata 

 to which it has been possible to ascribe a vasoconstrictor activity.^ 

 In accordance with the experiments of Dittmar,^ this center is bilateral 

 and lies about the middle of the fourth ventricle in the tegmental 



^ Owsjannikow, Ber. d. sachs. Gesellsch. d. Wissensch., xxiii, 1871. 

 * Ber. der sachs. Akad der Wissensch., math. phys. Klasse, xxv, 1873. 



