196 DISSECTION OF FROG. 



The vein almost black in colour accompanies it in the lower part 

 of its course, then leaves it above to run outwards. This vein after 

 joining a transverse branch higher up, goes to form the renal-portal 

 vein. 



Clear the nerve carefully from its surroundings, lifting it with the 

 glass seeker, and divide all restraining tissue around it with scissors. 



Never hold the nerve itself with forceps, and do not pinch or 

 pull it in any way. Remember that this is a living structure 

 and very easily injured. It should be kept moist with normal 

 saline and must on no account be allowed to dry. 



Two branches of the nerve will have to be cut in the thigh one 

 nearly midway externally, and one higher up upon its mesial aspect. 

 Observe that muscles twitch when this is done, owing to the mechanical 

 stimulation of the nerve when it is being cut. 



In severing the connective tissue where the nerve enters the abdominal 

 cavity, keep the points of the scissors up and near the iliac bone, as the 

 nerve lies more to the inner side. 



Cut through the coccigeo-iliacus muscle along the bone. 



Lift the end of the urostyle, clear it from the subjacent parts (con- 

 tents of the abdominal cavity) sever it from the spinal column. The 

 sciatic plexus of both sides are now in view. Cover this part up with 

 a flap of the skin and proceed to clear the lower end of the nerve. 



When the skin is used as a protective covering the outer 

 surface must not be placed in contact with muscle or nerve* 

 as its secretion is injurious to them. 



Near the knee the sciatic divides into an outer peroneal branch which 

 passes under a tendinous slip from the gastrocnemius muscle, and an 

 inner tibial branch which turns under the latter muscle to be distributed 

 to its under surface. 



Note : The frog is to be employed in its present condition 

 for the performance of those experiments which can be carried 

 out without recording. For recording purposes the nerve-muscle 

 preparation requires to be completed as follows : 



The Nerve-muscle preparation. (Fig. 30.) Where the Tendo 

 achillis passes round the heel it is thickened by a sessamoid cartilage. 



