STRUCTURE OF NERVE-CELLS 



79 



The ganglion if large is enclosed by an investing capsule of connective 

 tissue which is continuous with the epi- and peri-neurium of the 

 entering and issuing nerve-trunks. 



The structure of the nerve-centres and the arrangement of the 

 cells and fibres in them are given in Lessons XXXVI. to XXXIX. 



FIG. 96. LONGITUDINAL SECTION THROUGH THE MIDDLE OF A GANGLION ON 

 THE POSTERIOR ROOT OF ONE OF THE SACRAL NERVES OF THE DOG, AS 

 SEEN UNDER A LOW MAGNIFYING POWER. 



a, nerve-root entering the ganglion ; b, fibres leaving the ganglion to join the mixed spinal 

 nerve; c, connective tissue coat of the ganglion; d, principal group of nerve-cells, with 

 fibres passing down from amongst the cells, probably to unite with the longitudinally 

 coursing nerve-fibres by T-shaped junctions. 



Development. The cells and fibres of the nervous tissue are 

 developed from cells derived from the ectoderm or epiblast of the embryo. 

 The nerve-fibres are at first developed as pale fibres like the fibres 

 of Eemak ; it is uncertain whether they are formed by the coalescence 

 of a number of cells, or whether they grow out as the processes of 

 nerve-cells. The medullary sheath is subsequently added. 



When a nerve is cut, the fibres beyond the section as far as their 

 terminations undergo a process of degeneration, the medullary sheath 

 being broken up and the axis-cylinder interrupted and eventually 

 absorbed. New nerve-fibres are at length produced by a growth of the 

 axis- cylinders in the proximal end of the nerve. 



