502 Dynamic Theory. 



brain with the extremities of the body, are called the peripheric nerves. 

 All the peripheric nerves do not possess all the parts described above. 

 Some have no medullary sheath, and consist, therefore, of axis cylin- 

 ders immediately surrounded by the nerve sheath or neurilemma. 

 When many nerve fibres are united into a bundle, these marrowless 

 fibres are gray and more transparent, and are therefore sometimes called 

 gray nerve fibres. Those nerve fibres which have medullary sheaths, 

 appear more yellowish white. 



FIG. 236. Nerve fibres. (Shultze & Glaus.) 

 a. Non-Medu Hated Sympathetic Fibre. 

 &. Medullated Fibres, one of which lias com- 

 menced coagulation of the axis cylinder ( death of 

 nerve ). 



n c Medullated nerve fibre with the sheath of 

 Schwann. 



If the nerves are traced to the peri- 

 phery, more and more nerve fibres are 

 continually found to branch off from the 

 common stem, so that the branches and 

 branchlets gradually become thinner. 

 At last, only separate fibres are to be 

 seen, these being, however, still in ap- 

 pearance exactly like those constituting 

 the main stem. Such fibres, as up to 

 this point have had medullary sheaths, 

 FIG. 236. now frequently lose them, and therefore 



become exactly like gray fibres. The axis cylinder, or band itself, then 

 sometimes separates into smaller parts, so that a nerve fibre, thin as it 

 is, ramifies over a very large surface. The ends of the nerve fibres are 

 connected at their outer extremity sometimes with muscles and some- 

 times with glands, which they excite to action. Such nerves are called 

 motor nerves. Others are connected at their outer ends with the sense 

 organs eye, ear, tongue, &c. , and these are called sensory nerves. At 

 their inner ends they are connected with nerve-ceils, from which, if they 

 are motor nerves, they receive their stimulation, and to which, if they 

 are sensory nerves, they carry it. There are vast numbers of short 

 nerve fibres which connect the nerve cells with each other in the brain, 

 spinal cord and ganglions, or cell-knots, in different parts of the body. 

 The nerves in the brain and spinal cord are in the same variety of con- 

 ditions as the peripheral nerves, being possessed of all the parts or des- 

 titute of some one of them. Each ganglion cell is composed of the cell 

 body, the kernel or nucleus and the nucleolus. Some ganglion cells are 

 surrounded also by a membrane, which is a continuation of the neuri- 

 lemma of the nerve fibres that are connected with the cell. ' ' The 

 kernel is finely granulated, and is composed of a protoplasmic mass," 

 which, in its normal state, is nearly transparent. Some of the ganglion 



