786 Comparative Animal Physiology 



be counterbalanced by immersion in glycerin of the same refractive index, 

 whereupon a slight positive birefringence appears; these fibers are said to be 

 metatropic. 



Most invertebrate nerve fibers are said to be "non-myelinated," but in 

 many of them examination with polarized light reveals a thin outer layer 

 which is positively birefringent with respect to the radial axis.^^- ^'^^ Certain 

 fibers of crabs and shrimps, and of cockroaches and other insects, Nereis, etc., 

 are metatropic.^^'^- ^^^ The protein layer has a refractive index of 1.58 in fibers 

 from lobster cord, frog motor root, and cat corpus callosum.^^ The reversal 

 in sign in glycerin reveals a thin lipoid layer which in insects may even 

 stain with sudan dyes.^'*'^ This layer can be removed with fat solvents. As 

 the diameter increases, the positive birefringence decreases so that earth- 

 worm giant fibers are weakly myelotropic above 1 1 /x, and shrimp giant fibers 

 above 8 /x,^'-*^ as compared with frog fibers, above 2 /x.^^^ In giant fibers of the 

 prawn, Leander, the ratio of axis cylinder to total fiber diameter is 0.53 for 

 fibers smaller than 10 ^, 0.69 for 10-20 /x fibers, and 0.77 for 20-50 /x fibers.^o" 

 Squid giant fibers are uniformly positive with respect to length (negative to 

 radial axis) regardless of diameter, but when immersed in glycerin a lipoid 

 sheath is demonstrated which is less than 1 per cent of the fiber diameter.^^ 

 In summary, most invertebrate fibers, except large giant fibers of earthworm 

 and shrimp, resemble vertebrate thinly myelinated fibers in optical properties— 

 a thin lipoid sheath masked by protein. Some lipoid is probably present out- 

 side all nerve fibers. 



The fastest vertebrate fibers are of medium size (4 - 15 /x) but have thick 

 sheaths (30-50 per cent of total fiber diameter), whereas the fastest fibers 

 of invertebrates are large (50 to several hundred microns) but have thin 

 sheaths (1 — 10 per cent of the diameter— Table 76). In both these types 

 of fibers the ratio of axis cylinder to total fiber diameter usually increases with 

 increase in size of the fibers, up to some critical size. Taylor^^^ summarized 

 the relation between diameter, sheath, and velocity as follows: 4 /x fiber of 

 the cat saphenous at 38° conducts at about the same rate (25 M./sec.) as a 

 650 fx squid giant axon at 20°. Assuming a Qio of 1.5, an 8 /x cat saphenous 

 fiber would conduct at 20° at the same rate as the 650 /x squid giant fiber. 

 The myelin sheath of the mammalian fiber constitutes some 42 per cent of 

 its total diameter, whereas in the squid giant fiber the sheath is about 1 per 

 cent of the fiber diameter. 



Many invertebrate nerve fibers also have a connective tissue and glial 

 sheath. In some invertebrate fibers (prawn,'°^ squid^"*"*), connective tissue 

 cells lie between the thin lipoid sheath and the axon. There is no evidence 

 that the connective tissue sheath speeds conduction. 



Nodes. Some effects of polarization can be explained by the assumption 

 that impulses in frog medullated nerve travel in a saltatory fashion from node 

 to node.^^** There are regions of thinning of the sheath in shrimp giant 

 fibers.^*'^' ^'^'^ However, dependence on sheath and nodes for speed is largely 

 a vertebrate adaptation. Myelinated spinal tracts conduct rapidly but lack 

 nodes.^"^ 



Giant Fibers. There are two types of giant fiber systems. Each has ap- 

 parently evolved independently several times. The first type consists of a 

 single large or giant cell which gives rise to a single large axon. The second 



