THE TISSUES 75 



stains deeply with basic stains, and which seems to be used up 

 during the activity of the neuron, may accumulate in granules. 

 The granules formed of this material are generally known as 

 Nissl's granules (fig. 36). 



These cells give off at least one process, which continues for 

 some distance, as the axon. Frequently other processes are 

 given off, which may either pass away as fibres,, or may, while 

 still in close proximity to the cell, form a branching system of 

 dendrites. The axons end in much the same manner, so that 

 all the processes are essentially the same. These processes are 

 fibrillated, and the fibrillse may be traced through the protoplasm 

 of the cells (fig. 37). In many cases the dendrites show little 

 buds or gemmules upon their course, and, according to some 

 observers, it is through these 

 that one neuron is brought into 

 definite relationship at one time 

 with one set of neurons, and 

 at another with other adjacent 

 neurons. There is also some 

 evidence that the dendrites as a 

 whole may expand and contract, 

 and thus become connected with 

 those of adjacent neurons. 



Axon. The axon process, as it FIG. 37. A Nerve Cell highly 

 passes away from the cell, becomes magnified to show passage of 

 a Nerve Fibre, and acquires one P[ ocesses t!irough the proto ' 



plasm. 



or two coverings. 



1. A thin transparent membrane, the primitive sheath or 

 neurilemma, is present in all peripheral nerves. Between it 

 and the axis cylinder there are a number of nuclei surrounded 

 by a small quantity of protoplasm, the nerve corpuscles. The 

 mode of origin of these is unknown. Fibres with only this 

 sheath have a grey colour, and may be called non-medullated 

 fibres. They are abundant in the visceral nerves. 



2. A thick white sheath the medullary sheath or white 

 sheath of Schwann which gives the white colour to most of 

 the nerves of the body, appears somewhat late in the develop- 

 ment of many nerve fibres. It lies between the primitive 

 sheath with the nerve corpuscles and the axon. It is not 

 continuous, but is interrupted at regular intervals by con- 



