20 THE AMERICAN MONTHLY [Jan. 



note that they alone compose it, the muscle fibres dis- 

 appearing at the end. Examine single fibres with a 

 high power, and recognize, tliat they are composed of 

 etill smaller ^<5/'/7Za^ which run lengthwise in the fibre; 

 that there is a sheath enclosing the fibre, sarcolemma\ that 

 the fibrillae are marked with lines crossing them at equal 

 distances, and that this gives to the fibre a cross-mark- 

 ing, ^/r/af/ow. Directly beneath the sarcolemma there are 

 elongate granular cell-7iuclci, these may not be easily rec- 

 ognized in the glycerine preparation unstained. If so 

 etain a second preparation before the ajiplication of gly- 

 cerine with borax carmine, decolorize with acidulated 

 alcohol and examine small fibres for nuclei, note their 

 exact size and position with reference to the fibre. 



12. The NERVOuy System. — In dissecting the dorsal 

 wall of the body cavity next the spinal column you have 

 probably noted white threads running in the lines be- 

 tween the myotomes outward from the spine, these are 

 the spinal nerves. A pair can be seen at the interval be- 

 tween each two vertebrae through the cMitire lengtli of 

 the trunk, and tiiey are also present in the post-abdomen 

 In the same way, though not there so easil y traced; there is 

 thus a metamerism in the nervous system. The spinal 

 nerves are of approximately the same diameter through- 

 out the series exce[)ting at the levels of the front and 

 hind limbs, where several of them are ct>nsiderably 

 larger than the rest, this is because Ihey are com- 

 posed of the additional fibers that go to the muscles and 

 skin of the limbs. How many of these nerves to the 

 limbs do you recognize? In the head there is a seiiesof 

 cranial nerves which relate the jiartsof the head with the 

 brain; as in the fish, the sj)inal canal lodges the spinal 

 ford^ which can be seen by removing tlie neural arches. 

 There is a sympathetic system but its dissection is very dif- 

 ficult. 



