148 THE TISSUES 



from the syncytium and becoming amoeboid. Such amoeboid cells 

 may convey various products of degeneration to the vicinity of the 

 perivascular lymph spaces. There may also result a temporary in- 

 crease in the glial fibres (gliosis). 



According to good authorities a continuous glial membrane forms 

 the outer boundary of the ectodermal elements of the central nervous 

 system, i.e., the nerve tissue and glia. This glial membrane is of 

 course in apposition with the mesodermic connective tissue which 

 forms the pia or inner covering of the brain and cord and also with 

 the connective tissue of the adventitia of the blood-vessels which 

 penetrate the central nervous system. The glial membrane is 

 formed by superficial flattened cells and processes of other glial cells 

 which come into contact with the connective tissues. The lining 

 of the cavity of the neural tube is formed by the ependymal cells 

 which are a form of neuroglia cells. These cells are columnar epi- 

 thelial cells each with a process penetrating a variable distance into 

 the wall of the neural tube and joining the general neuroglia syncy- 

 tium. Their nuclei lie near the central cavity of the tube and they 

 contain glia fibres. The inner fining of the chorioid plexuses of the 

 brain is a single layer of cuboidal epithelial cells which are believed to 

 have a secretory function and contribute to the production of the 

 cerebro-spinal fluid. It is probable that the ependyma cells also 

 have a secretory activity. 



The neurilemma cells also probably originate from the neural 

 tube. If this is the case it is evident that they may be regarded as 

 a special form of neuroglia cell. It has already been seen that in 

 pathological changes they behave similarly to the neuroglia cells 

 in the central nervous system. 



TECHNIC 



(i) Pieces of the cerebral cortex are stained by one of the Golgi methods. 

 If the rapid or mixed silver method is used, sections must be mounted in hard bal- 

 sam without a cover; if the slow silver or the bichlorid method is used, the sec- 

 tions may be covered. Sections are cut from 75 to loo/^ in thickness, cleared in 

 carbol-xylol or oil of origanum and mounted in balsam. This section shows only 

 the external morphology of the neurone. It is also to be used for studying the 

 different varieties of neurogha cells as demonstrated by Golgi's method (see 

 page 147). 



(2) Thin transverse slices from one of the enlargements of the spinal cord 

 are fixed in absolute alcohol. Thin sections (5 to lo/t) are stained by Nissl's 

 method (page 39) and mounted in balsam. This section is for the purpose of 



