STRUCTURE OF THE NERVE-CENTRES. 



517 



of Iloyer, Lipmann and others, branching nerve-fibres pass to the nucleoli 

 of the corneal corpuscles and to the nucleoli of the cells of the posterior 

 layer of epithelium. 



Structure of the Nerve-centres. A peculiar pigmentary matter in the 

 nerve-cells and in the surrounding granular substance gives to the nerve- 

 centres a grayish color, by which they are readily distinguished from the 

 white, or fibrous division of the nervous system. Wherever this gray matter 

 is found, the anatomical elements of the tissue are cellular, except in the 

 nerves formed of gray, or gelatinous fibres. Under the general division of 

 nerve-centres, are included, anatomically at least, the gray matter of the 

 cerebro-spinal centres, the ganglia of the roots of the spinal and certain of 

 the cranial nerves, and the ganglia of the sympathetic system. In these 

 parts are found cells, which constitute the essential anatomical element of 

 the tissue, granular matter resembling the contents of the cells, pale fibres 

 originating in prolongations of the cells, elements of connective tissue, deli- 

 cate membranes enveloping some of the cells, with blood-vessels and lym- 

 phatics. The most important of these structures, 

 in their physiological relations, are the cells and 

 the prolongations by which they are connected with 

 the nerves and with each other. 



Nerve-cells. The following varieties of cells ex- 

 ist in the nerve-centres and constitute their essential 

 anatomical elements ; viz., unipolar, bipolar and 

 multipolar cells. These cells present great differ- 

 ences in their size and general appearance, and some 

 distinct varieties are found in particular portions of 

 the nervous system. Unipolar and bipolar cells are 

 found in the ganglia of the cranial nerves and in 

 the ganglia of the posterior roots of the spinal 

 nerves. Small unipolar cells are found in the sym- 

 pathetic ganglia. Multipolar cells present three or 

 more prolongations. Small cells, with three and 

 rarely four prolongations, are found in the posterior 

 cornua of the gray matter of the spinal cord. From 

 their situation they have been called sensory cells. 

 They are found in greatest number in parts known 

 to be endowed exclusively with sensory properties. 

 Large, irregularly shaped multipolar cells, with a 

 number of poles, or prolongations, are found chiefly 

 in the anterior cornua of the gray matter of the 

 spinal cord, and these have been called motor cells. 

 They sometimes present as many as ten or twelve 

 poles. 



Unipolar cells, such as exist in the ganglia of FIG. iss. Unipolar ceil from the 



. o c Gasserian ganglion (Schwalbe). 



e nerves as distinguished from the ganglia of the N , N , N , nuc iei of the sheath ; T , 

 cerebro-spinal axis, have but a single prolongation, ^branching at a node of 



