170 THE BLOOD AND ITS CIRCULATION. 



compressed by a strong expiration, so as to empty it com- 

 pletely, and bring its coats into close contact, we may 

 succeed in stopping the beating of the heart. This explains 

 those curious instances of persons who are able at will, to 

 stop the motion, arid consequently, the pulsation of their 

 heart. (See respiration.} 



Vessels. The vessels which we know contract under 

 direct excitation (heat, cold, shock, etc.), are also, in this 

 respect under the control of the nervous system. Cl. Ber- 

 nard has demonstrated that effects of this kind belong 

 especially to the province of the great sympathetic (vaso- 

 motor nerve), which sometimes produces contraction and 

 sometimes paralysis of the muscular coats of the vessels. 

 Some of the cerebro-spinal nerves produce the same effect. 

 Thus the chorda tympani paralyzes the arteries of the 

 sub-maxillary gland. These phenomena of contraction 

 or dilatation of the vessels have great influence on the 

 calorification of the organs in which they take place : they 

 are for the most part of a reflex nature, and are the conse- 

 quence either of an impression made upon sensory nerves, 

 or of some mental excitement (redness or paleness of the 

 face under the influence of the passions). The innervation 

 of the vessels thus offers the closest resemblance to that of 

 the heart. 



The physiology of the great sympathetic as a vaso-motor 

 nerve offers great difficulties, not only in this general point of 

 view, but also in that of its influence on the vessels, the origin 

 of its nerve filaments, and of their course and relation to the 

 nerves concerned in the "animal" processes or functions (vie de 

 relation) . 



After Henle had discovered smooth muscular elements in 

 the coats of the arteries, Stilling found nerves which disap- 

 pear in these coats, and gave these the name of vaso-motor 

 nerves, seeking to complete the anatomical fact by a physio- 

 logical hypothesis. Physiological researches on the subject, 

 however, only date as far back as 1851, when Cl. Bernard 

 showed that section of the great sympathetic nerve in the 

 neck of a rabbit produces considerable increase of temper- 

 ature in the ear of the corresponding side ; though at first 

 tempted to ascribe this phenomenon merely to a calorific 

 and direct action of the nerves, he soon saw that the heat of 

 the ear was simply due to a dilatation of the blood-vessel, 

 and to a greater afflux of blood; and showed, simultaneously 

 with Brown-Sequard, that by galvanizing the cephalic ex- 



