94 PRACTICAL HISTOLOGY. 



Transverse sections of fresh muscle may be readily 

 made with the freezing microtome. 



146. Examine (L.) an L. S. of muscle with its blood- 

 vessels injected. The capillaries are seen to run mostly 

 parallel with the fibres. These longitudinal vessels are united 

 by short transverse branches. 



147. With regard to the examination of muscle by polarised light 

 ( 53), the memoirs by Briicke (Strieker's Histology ; vol. i. p. 235) and 

 Schaefer ("Minute structure of the Leg-Muscles of the Water- Beetle," 

 Philosophical Trans., 1873) may be consulted. 



NERVE TISSUE. 

 NERVE FIBRES. 



148. White Nerve Fibres (H.) Place the lumbar or 

 sciatic nerves of a frog in \ per cent osmic acid for two 

 days. Wash in distilled water for a day. Tease with 

 needles, and preserve in Farrants' solution. 



Observe in each fibre a central somewhat clear band the 

 axial cylinder. At the broken end of a fibre this band 

 may be found projecting. It is apparently homogeneous, 

 but in the spinal cord it may be seen that at the junction of 

 the axial cylinder with a nerve cell it is fibrillated. Therefore 

 the axis is believed to be a bundle of nerve fibrils held to- 

 gether by an interstitial cement. Outside the axis there is the 

 medullary sheath, or white substance of Schwann darkened by 

 the osmic acid (for convenience we term this the white sheath). 

 In the cerebro-spinal nerves but not in the brain and 

 spinal cord the medullary sheath is enclosed in a delicate 

 homogeneous membrane, the primitive sheath of Schwann 

 (for convenience we term this the gray sheath). It is difficult 

 to see this, but it may perhaps be found prolonged from 

 the broken end of a fibre. On tracing the fibres lengthwise, 

 Ranvier's nodes will readily be found. Each node is a 

 slight constriction, at which the white sheath is deficient. 

 The axial cylinder, however, extends through the node, so 

 that the continuity of this the essential part of a nerve 

 fibre is intact. These nodes which are only to be found 



