NERVOUS TISSUES AND SYSTEMS. 81 



Fibres. A nerve fibre is derived from an axis cylinder of a gan- 

 glion cell. A ganglion cell is a cell of the first-type, producing a 

 medullated fibre. The sheath which invests the whole fibre is the 

 neurilemma, or primitive sheath. It is a thin, transparent, tough 

 membrane of areolar connective tissue. Directly beneath the neuri- 

 lemma is the medullary sheath, which consists of a semi-fluid, high- 

 ly-refracting substance, called myelin. Beneath the medullary 

 sheath and immediately surrounding the axis cylinder is a delicate 

 and very thin investment, the axilemma. The axis cylinder is the 

 essential part of the fibre. It may be naked, or covered with medullary 

 sheath alone, or with neurilemma alone, or with both. The medul- 

 lary sheath is absent in the nerves of the sympathetic system and 

 occasionally in those of the cerebro-spinal system. The neurilemma 

 is absent in the fibres which traverse the brain and spinal cord. It 

 is also absent in the fibres of the olfactory nerves. At certain reg- 

 ular intervals, the medullated fibres lose their medullary sheath, and 

 the neurilemma closes down upon the axis cylinder. These points 

 are called the nodes of Ranvier. The space between two nodes is 

 the internode. About midway of the internode, just beneath the 

 neurilemma, is the nerve corpuscle. 



The non-meduliated fibres occur in the sympathetic system. Each 

 fibre consists of axis cylinder (composed of a bundle of fibrils), the 

 neurilemma, and an oval nucleus upon the surface of the fibre. 



II. NERVOUS SYSTEMS. 



There are three important nervous systems central nervous sys- 

 tem, sympathetic system, and terminal system. 



CENTRAL NERVOUS SYSTEM. 



This is sometimes called the cerebro-spinal system. It consists 

 of the spinal cord and brain. Each of these is constituted of gan- 

 glion cells, nerve fibres, neuroglia, and connective tissue. 



SPINAL CORD. 



The spinal cord, located in the spinal column, consists of gray 

 and white nervous matter. The gray matter is in the center of the 

 cord, and the white matter surrounds it. The gray matter is made 



