Nebvotjs Tissue 51 



in a network of another substance called neurokeratin. Some histologists. 

 have described a fourth type of sheath. 



It is probable that all fibers have a neurilemma, except those in the 

 ' gray matter ' of the nerve centers. In all cases, however, the neuri- 

 lemma is absent for a short distance after leaving the cell-body, and again 

 at the farthest end. Most observers claim that fibers in the white matter 

 of the brain and cord have no neurilemma: Ramon y Cajal, however, finds 

 the neurilemma on some of those fibers. All fibers in the ' white ' matter 

 of the brain and cord, and most fibers elsewhere, have the myelin (and 

 neurokeratin) sheath for a portion of their course, at least, and are called 

 mcdullated fibers. The fibers which have not the myelin sheath are called 

 non-medullated, these latter belonging chiefly to the visceral system. 



The myelin sheath, when it is present, always lies next to the axon, 

 under the neurilemma (if the latter is present). The connective tissue 

 sheath, which is found only in peripheral fibers, is outside the neurilemma. 



Medullated nerve fibers have the myelin deposit interrupted annularly 

 at intervals of from 80/u. to a millimeter along it; the interruptions are 

 known as the nodes of Ranvier [Fig. 35] ; all branches of medullated 

 fibers appear at these nodes. As there is but one nucleus for every inter- 

 nodal segment of the myelin sheath, such segments are thought by some 

 histologists to be each developed from a single cell, the neurilemma cor- 

 responding to the cell-membrane, and the neurokeratin to the spongio- 

 plasm. 



Outside of the brain and nervous system there are characteristic group- 

 ings of cell-bodies and fibres of neurons, although not all fibres and cell- 

 bodies are associated in these groups. The groups of cell-bodies are 

 called ganglia; and the groups or bundles of fibres are called nerves. 

 Many nerve fibres follow (or are contained in) one nerve-bundle for 

 a part of its course, afterwards leaving it to join other nerve-bundles. 



Nerves are spoken of as medullated and non-medullated, according as 

 they are made up of medullated or non-medullated fibers. The larger 

 medullated nerves are made up of a number of bundles (called funiculi 

 or fasciculi), each surrounded by a sheath of dense connective tissue 

 (perineurium) which is continuous with the sheath of Henle surround- 

 ing each fiber, the whole nerve being surrounded by a layer of loose con- 

 nective tissue, the epineurium [Fig. 36]. 



The spinal cord lies in the cervical, thoracic (or ' dorsal ') and lum- 

 bar portions of the spinal canal of the vertebral or spinal column [Fig. 

 38]. The spinal column is composed of 33 vertebrae [Fig. 39] articu- 

 lated together, each vertebra, except the lower, false or fixed vertebrae, 

 having back of its ' body ' a vertical opening, somewhat annular in cross- 



