350 GENERAL DIRECTIONS. 



make on the central nervous system are contained in the two 

 last chapters. 



No special chapters on the senses have been introduced, as 

 there seemed to be no mean between the common simple 

 observations on the one hand which are found in all the text- 

 books and such elaborate instructions on the other as would 

 hardly come under the scope of this work. 



CHAPTER XIX. 

 GENERAL DIRECTIONS. 



I. The Nerve-Muscle Preparation. Having pithed a 

 frog and destroyed both its brain and spinal cord, lay it on its 

 belly and make an incision through the skin along the middle 

 line of the back of the thigh, from the knee to the end of the 

 coccyx, and carry the incision along the back about midway 

 between the coccyx and ileum (fig. 266, line k, m, n). On 

 drawing back or removing the skin, there will come into view, 

 on the outside of the thigh, the triceps femoris (fig. 267 a), 

 on the median side the semi-membranosus c, and between these 

 the small narrow biceps femoris b. With the " seeker" sepe- 

 rate gentty b and c ; the sciatic nerve and femoral vessels will 

 be found running between them. Gently tear away, with the 

 seeker, the connective tissue round the nerve, beginning near 

 the knee (where it divides into two branches), and working 

 upwards till the muscle n is reached. Be careful to touch the 

 nerve itself as little as possible, and on no account seize it 

 with a pair of forceps. Carefully cut through the pyriform 

 muscle n and the connective tissue in which the nerve is em- 

 bedded at this point, divide the iliac-coccygeal muscle rf, right 

 through, and follow the three nerves (which come into view 

 when the muscle is removed, and which go to form the sciatic 

 and other nerves) right up to the vertebral column. Cut the 

 column across just above the last lumbar vertebra, and bisect 

 lengthways the piece so cut off. Hold the bony fragment witli 

 the forceps, lift it up and free it from the tissues around, and 

 then follow with the scissors the nerves right down to the 

 knee, cutting away their various branches and removing any 

 tissue which still may be clinging to them. 



Now remove the skin from the leg; the gastrocnemius (fig. 

 267 g) will at once be recognized : cut through the tendo 

 Achillis at /, below the thickening at the heel. Holding the 

 cut tendon with a pair of forceps, it will be easy, with a few 



