254 CIRCULATION OF THE BLOOD. 



injured limb, the key is opened. The same series of phe- 

 nomena present themselves as before contraction and slowing 

 of the circulation, preceded by a much less obvious accelera- 

 tion. If now the other web is brought under the microscope, 

 it is seen that the contraction of the arteries is very incqnsider- 

 able, the acceleration is more distinct. The explanation of 

 this is easy. The sciatic nerve being the channel by which 

 most of the vasomotor fibres find their way to the arteries of 

 the web, those vessels are in great measure (but not entirely) 

 paralyzed by its division. Consequent!}^ of the three effects 

 produced by excitation of the vasomotor centre viz., increased 

 vigor of the contractions of the heart, increase of arterial press- 

 ure, and contraction of the arteries the first two only mani- 

 fest themselves in acceleration of the blood-stream. In the 

 other limb, the vasomotor nerves being intact, the phenomena 

 present themselves in their completeness. The effect of 

 direct and indirect excitation of the medulla on the vessels of 

 the mesentery has as yet been imperfectly investigated. It is 

 certain that in general the contraction of the mesenteric 

 arteries is much less marked than of those of the web. It is 

 often entirely absent, the only change observed during excita- 

 tion being that the stream is accelerated. These facts do not 

 indicate that these arteries are out of the control of the 

 cerebro-spinal centres, but merely that the nerves excited are 

 not in reflex relation with them. 



52. Reflex Excitation of the Medulla Oblongata in 

 Mammalia. The vasomotor centre may be stimulated in 

 the dog, rabbit, or cat, by the electrical excitation of any sen- 

 sory nerve. The most convenient for the purpose is the sciatic. 

 The requirements are the same as for an ordinary kymographic 

 observation. If it is intended to excite the trunk of the sciatic 

 nerve, the animal must rest on its side. It must first be ren- 

 dered insensible by opium or chloral, and subsequently curar- 

 ized. In order to expose the sciatic nerve, an incision must 

 be made from a point half way between the trochanter and 

 the promontory of the ischium towards the tendon of the 

 biceps. Such an incision runs nearly parallel to the inner 

 and posterior edge of the long head of the muscle just named, 

 which edge must be found and drawn outwards. In the upper 

 third of the thigh, the nerve lies between the biceps and the 

 adductor magnus, further down, between the biceps and the 

 semi-membranosus. If it is desired to stimulate the nerve 

 near its distribution, the peronaeal nerve may be found very 

 readily in front of the ankle, on the fibular side of the com- 

 mon extensor of the toes. It is often called the n. dorsalis 

 pedis. 



Excitation of the central end of the divided sciatic or of the 

 peronseal nerve produces effects which are indistinguishable in 



