ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. G05 



Sometimes, again, it is seen under the aspect of reticulated plates 

 bounding the space in which the nerve-fibars pass. This is beautifully 

 demonstrated in the white substance of the columns of the spinal cord. 



Elsewhere the neuroglia is found to be a homogeneous, gelatini- 

 form substance, as in the ependyma of the spinal cord or in the gela- 

 tinous substance of Eolando in the postero-lateral groove of the same 

 structure. 



Besides the fibers and plates already mentioned, neuroglia con- 

 tains cells. These are star-shaped, flat, and nucleated. They have 

 numerous prolongations. By the aid of these prolongations the cells 

 of the neuroglia anastomose freely with one another to form a very 

 complicated network. This incloses in its meshes the nerve-elements. 



Neuroglia enjoys the role of a true cement which unites all of 

 the fibers and nerve-cells. 



Classification of Nerve-cells. According to Schafer, nerve-cells 

 are broadly classified into: "1. Afferent cells, which receive impres- 

 sions at the periphery to convert them into impulses. The latter then 

 pass toward the central nervous system. 2. Efferent cells, which send 

 out nervous impressions toward the periphery. 3. Intermediary cells, 

 which receive impressions from afferent cells to transmit them directly 

 or indirectly to efferent cells. 4. Distributing cells, which occur near 

 the periphery, and, receiving impulses from efferent cells, distribute 

 them to involuntary muscles and secreting cells. The cells of this 

 class belong to the so-called sympathetic system. 



"The afferent and efferent cells are known as root-cells. The 

 greater number of the nerve-cells of the brain and cord belong to the 

 intermediate class. They serve the purposes of association and 

 coordination and afford a physical basis for psychical phenomena." 

 Efferent fibers are also called cellulifugal. Afferent fibers are also 

 called cellulipetal. 



Structure of the Gray Substance. The gray matter is formed 

 (1) of nerve-cells, (2) of neuroglia-cells, (3) of fibril elements repre- 

 senting the prolongations of nerve- and neuroglia- cells, (4) of an 

 intervening network formed by the branching fibrils, and (5) of 

 )lood-vessels. Elements 1, 2, and 3 (here enumerated) of the struc- 

 ture have been treated previously in detail. 



The blood-vessels penetrate the gray substance, and are sur- 

 rounded with a layer of connective tissue coming from the pia mater, 

 rhich they have received in their passage along and through this 

 lembrarie. The connective tissue forms sheaths around the capillary 

 network, arterioles, and little veins, in which the vessels seem to float. 



