NEUKOGLIA. 



511 



j norphological difference between a dendrite and an axon disappears, and van 



Tehuchten's functional distinction alone remains characteristic, viz., that the axon 



s cellulifugal and conducts 'impulses away from the cell, whilst the dendrites are 

 j ellulipetal and conduct impulses towards the cell. 



It is, however, more in accordance with the facts to regard the sensory neurones 

 . is genetically quite distinct from the rest of the nervous system (see p. 498). 



Neuroglia. The neuroglia is the supporting tissue of the cerebro -spinal axis. 



!t may be considered to include two different forms of tissue, viz., the lining 



jpendymal cells and the neuroglia proper. We place these under the one heading, 



seeing that they have a common developmental origin. 



The ependymal cells are the columnar epithelial cells which line the central 



ianal of the spinal medulla and the ventricles of the brain. In the embryonic con- 



lition a process from the deep extremity of each cell, traverses the entire thickness 



>f the neural wall, and reaches the surface. It is not known whether this process 

 !3xists in the adult. 



The neuroglia proper is present in both the white and the gray matter of the 



ierebro-spinal axis. It constitutes an all- j 



3ervading basis substance, in which the 



/arious nerve elements are embedded in such 

 ii way that they are all bound together into 



i consistent mass, and are yet all severally 

 , isolated from each other. Neuroglia consists 

 of cells and fine filaments. The fibrils are 



present in enormous numbers, and by their 



interlacements they constitute what appears 



bo be a fine feltwork. At the points where 

 :t)he fibrils intercross may be seen the flattened 



jlial cells. Whilst the neuroglia is for the 

 i most part intimately intermixed with the 



nerve elements, there are, in both brain and 



spinal medulla, certain localities where it is 

 i spread out in more or less pure layers. Thus, 



upon the surface of the brain and of the 



spinal medulla there is such a layer ; likewise 



beneath the epithelial lining of the central 



3 analandof the cavities of the brain there 



is a thin stratum of neuroglia. 



The ependymal cells are derived from 



*'* 



EMBRYO, SHOWING EPENDYMAL AND NEUK- 

 OGLIAL CELLS (after v. Lenhossek). 



the original neuro-epithelial cells of the early . A > ^J, 1 ce "; B> f euroglial f)' _ 



, . r , , .,., ,, J [Note that the dorsal (posterior) aspect is below.] 



neural tube, and in all probability the neur- L 



oglia proper has a similar origin. They both, therefore, are products of the ectoderm. 



Summary. 1. The cerebro-spinal nervous system is composed of two parts, viz., 



. (a) a central part, consisting of the brain and spinal medulla, with the efferent nerve- 



1 fibres which pass out from them; (6) the ganglionic part, with the afferent nerve-fibres. 



2. Each of these parts has a different origin, and is composed of neurones which 

 I possess characteristic features. 



3. The ganglionic neurones are derived from the primitive cells of the neural 

 crest, and have each one process, which divides into two. Of these the central 

 division enters the cerebro-spinal axis, whilst the peripheral division becomes con- 



' nected with a peripheral part. The central fibres from the ganglionic cells in 

 the region of the spinal medulla form the dorsal or posterior roots of the spinal 

 nerves. The cells of origin of these posterior roots are outside the spinal medulla, 



i and carry impulses into its substance. 



. The cerebro-spinal neurones are derived from the neuroblasts in the wall 

 of the early neural tube. Certain of these furnish efferent nerve-fibres, which 

 issue from the spinal medulla in separate bundles termed the anterior or ventral 

 roots of the spinal nerves. In the case of the cerebral nerves, however, with the 



i exception of the trigeminal and facial nerves, the efferent fibres are not thus 



i separated from the afferent fibres at their attachment to the brain. 



