252 CIRCULATION OF THE BLOOD. 



below the occipital foramen. For this purpose, the occipital 

 bone must be perforated with a small trephine (fig. 203, d) in 

 the middle line between the occipital protuberance and the 

 occipital spine (see fig. 224). By this opening, a thin-bladed 

 knife is introduced in the middle plane, with its edge outwards, 

 by which the medulla is divided, first on one side, then on the 

 other. If the division is made as much as five millimetres 

 above the calamus scriptorius, the diminution of arterial press- 

 ure produced is as great as after section outside of the 

 cranium. In experiments in which the division was made 

 higher, the effect was found to be lessened, disappearing when 

 a point was reached about a millimetre below the corpora 

 quadrigemina. 



EXPERIMENTS RELATING TO THE REFLEX EXCITATION OF THE VASO- 

 MOTOR CENTRE. 



The vasomotor centre, although constantly in activity, may 

 be stimulated by impressions received by it through afferent 

 nerves. This can be shown both in the frog and in mammalia. 



51. Reflex Excitation of the Medulla Oblongata in 

 the Frog. For this purpose, the nerves in question may be 

 excited either with the aid of the ordinary excitor (fig. 225), 

 or by the application of a metallic brush to the skin. In the 

 latter case, one of the wires which form the secondary circuit 

 ends in a point which is inserted into the muscles; the other, 

 in the brush which is kept in contact with the skin in the im- 

 mediate neighborhood. The effect should be observed in the 

 web, in the mesenteiy, and in the great vessels leading to the 

 heart. The currents employed must be feeble when the nerves 

 are excited \)y the direct application of the electrodes to the 

 sensory nerves, but strong when it is intended to excite their 

 cutaneous or mucous endings. The periods of excitation 

 should always be very short. The experiment may be varied 

 as follows: a. A frog having been carefully curarized, with 

 the same precautions as were recommended for studying the 

 effect of direct excitation of the medulla, and arranged for the 

 microscopical observation of the circulation in the web, the 

 points of the excitor arc placed upon the tongue, the mouth 

 being kept open for the purpose. On opening the key, the 

 same changes exactly arc observed in the vessels as are pro- 

 duced by direct excitation. At the first moment the blood- 

 stream in the arteries is accelerated, but immediately after, 

 the arteries begin to contract sensibty. The contraction 

 increases gradually but rapidly for one or two seconds, and is 

 attended with slowing, and finally with arrest, of the circula- 

 tion. A maximum of narrowing having been attained, the 

 effect passes off as it came on. Even if the excitation is 



