152 STRUCTURE OF THE NERVOUS SYSTEM, BY MAX SCHULTZE. 



'inwards, affecting the homogeneity of the mass, and altering 

 the originally transparent highly refractive fibre in a very 

 peculiar manner (see woodcut 20, d). The change is accelerated 

 by the addition of water, whilst it is delayed for many hours 

 by immersion in solution of iodine in serum. The addition of 

 a solution of perosmic acid hinders the coagulation of the 

 medullary substance of the nerves in a similar manner, and 

 moreover colours it of an inky black. 



The medullated fibres of the nerve centres are imbedded in 

 an extremely delicate tenacious spongy connective substance, 

 the peculiar consistence of which preserves the fibres from 

 injury, notwithstanding their softness, and the absence of any 

 proper investing sheath. 



The medullated fibres of the peripheric nerves, on the other 

 hand, with the exception, perhaps, of those belonging to the 

 optic and acoustic nerves, each possess in addition, and external 

 to their medullary sheath, a special investment of connective 

 tissue, constituting the so-called sheath of Schwann. This is 

 either a structureless, perfectly transparent, delicate membrane, 

 agreeing in its consistence and chemical constitution with the 

 sarcolemna of muscular fibre, or consists of several layers of 

 fibrillar connective tissue, and as in this presents nuclei scat- 

 tered through its substance. If the membrane be very thin, 

 the appearance of the medullated fibres is not materially altered 

 by its presence. The refractive external border of the medul- 

 lary sheath entirely prevents the extremely thin feebly 

 refracting sheath of Schwann from being seen. But the 

 resistance and firmness of the several nerve fibres are extra- 

 ordinarily augmented by it, and the facility with which long 

 portions of the fibres may be isolated in the peripheric nerves 

 depends essentially on its presence. It prevents also the 

 occurrence of the phenomena due to imbibition of fluids by 

 the medullary portion of the nerves, as well as the formation of 

 varicosities which are so characteristic of the medullated fibres 

 of the nervous centres (fig. 19). The extraordinary differences 

 in appearance and consistence exhibited by fibres of equal size, 

 and similarly composed of axis cylinder and medullary sheath, 

 but belonging respectively to the central and peripheric por- 

 tions of the nervous system, are essentially referrible to the 



