98 EXERCISE XVI 



succession; the posture of the knee becomes cumulatively more extended. 

 The flail-like drop of the leg below knee is checked by a remainder 

 of contraction, and the fall may excite a second though smaller jerk-contraction, 

 giving a brief clonic reaction in response to the single tap on tendo 

 patellae. The jerk does not terminate, therefore, in any free pendular swing, 

 as it does in the decapitate preparation. The form of the jerk now present 

 is the tonic form, characteristic of the bulbo-spinal or decerebrate preparation. 

 The reason is that the knee-jerk muscle, vastocrureus, is with other extensor 

 muscles exhibiting postural (tonic) contraction (cf. exerc. XIX). 



Obs. 76 a. ly^ I Operation for :2nd cervical ganglion. (PI. VIII, figs. 1. 2, 4.) Place the 



Tli6 * law ' of . i^ 1/ \ ' o - 7 / 



the roots tested pi'^paration prone, with a wooden block under the neck, and a hook- weight 



by pinna reflex, hung on the string which has been attached to the skin over occiput stump ; or 



the mandible-clamp can be fixed by a ball and socket joint clamp to a short 



strong vertical standard. The front end of the neck is thus drawn forward, 



somewhat opening the space between atlas and axis vertebrae (in which space 



the 2nd cervical ganglion lies) without embarrassing the circulation and the 



ventilation of the lungs. 



Skin incision along mid-dorsal line from occiput for 5 cm. backward. At 

 posterior end of this a transverse incision extending for 2-5 cm. to either side 

 of it. Reflect laterally the skin-flaps and retract them by clip-weights. 

 Clavio-trapezius muscle is thus exposed, and overlapping it, in front, a small 

 triangular piece of levator auris longus, and, behind, a small piece of acromio- 

 trapezius muscle. 



Cut through these muscles in the mid-line. Feel with the finger, in the 

 median interval between their cut edges, the ridge of the spinous process of 

 axis. Cut through the muscles next underlying, biventer cervicis, both along 

 median line and along posterior edge of the wound, and retract the freed 

 portions laterally. 



Two longitudinally running slug-shaped muscles, the recti capitis post, maj., 

 are then seen one to either side of the mid-line. Incise carefully between them 

 and reflect them laterally from the surface of the spinous process of the axis. 

 This has to be done with care because they cover the ganglion of the 2nd 

 cervical nerve of either side (PL VIII, fig. 2) near the base of the beak of the 

 spinous process of axis. The ganglion is exposed as the muscles are being freed 

 and drawn aside ; the overhanging beak of the spinous process is at the same 

 time laid bare. Make out the fibrous membrane between the posterior edge 

 of the lamina of atlas in front and axis behind, and note the root of 2nd 

 cervical nerve lying on and plunging through that membrane to reach the 

 spinal cord itself. Clear the muscles from the median part of the lamina of 



