COMPOSITION OF THE NERVOUS SUBSTANCE. 583 



ment of the spinal cord, by Dean, shows the mode of connection between certain of the 

 cellular prolongations and the fibres of the anterior roots, and the commissaral fibres by 

 which the cells are connected with each other. 



Accessory Anatomical Elements of the Nerve-centres. While we must regard the cells 

 of the gray matter and the axis-cylinder of the nerves as probably the only anatomical 

 elements concerned in innervation, there are other structures in the nervous system 

 which it is important for us to study. These are the following : 1, Outer coverings sur- 

 rounding some of the cells ; 2, intercellular, granular matter ; 3, peculiar corpuscles, 

 called myelocytes ; 4, connective-tissue elements ; 5, blood-vessels and lymphatics. 



Certain of the cells in the spinal ganglia and in the ganglia of the sympathetic system 

 are surrounded with a nucleated covering, removed a certain distance from the cell itself, 

 so as to be nearly twice the diameter of the cell, which is continuous with the sheath of the 

 dark-bordered fibres. This membrane is always nucleated, and Kolliker has lately shown 

 that it is not homogeneous, as was at one time supposed, but is composed of a layer of 

 very delicate epithelium. The physiological significance of this covering is not apparent. 



In the gray matter of the nerve-centres, there is a finely granular substance between 

 the cells, which closely resembles the granular contents of the cells themselves. In addi- 

 tion to this granular matter, Robin has described new anatomical elements which- he has 

 called myelocytes. These are found in the cerebro-spinal centres, forming a layer near the 

 boundary of the white substance, and they are particularly abundant in the cerebellum. 

 They exist in the form of free nuclei and nucleated cells, the free nuclei being by far the 

 more numerous. The nuclei are rounded or ovoid, with strongly-accentuated borders, 

 are unaffected by acetic acid, finely granular, and generally without nucleoli. The cells 

 are rounded or slightly polyhedric, pale, clear, or very slightly granular, and contain 

 bodies similar to the free nuclei. The free nuclei are from ^Vs to innr f an mcn m 

 diameter, and the cells measure from ^-gVo * Wiro> an( ^ sometimes -j^Vo" f an mcn - 

 These elements also exist in the second layer of the retina. 



There has been a great deal of discussion with regard to the presence or absence of 

 connective-tissue elements in the cerebro-spiual centres. In the other ganglia, there has 

 never been any doubt with regard to the presence of connective tissue in greater or less 

 amount, and in the cerebro-spinal centres there can be hardly any question of the exist- 

 ence of an exceedingly delicate stroma, chiefly in the form of stellate, branching cells, 

 serving, in a measure, to support the nervous elements. 



The blood-vessels of the nerve-centres form an exceedingly graceful capillary net-work 

 with very large meshes. The gray substance is much richer in capillaries than the white. 



A remarkable peculiarity of the vascular arrangement in the cerebro-spinal centres 

 has already been described in connection with the lymphatic system. The blood-vessels 

 here are surrounded by what have been called perivascular canals, first described by 

 Robin, and afterward shown by His and Robin to be radicles of the lymphatic system. 



Composition of the Nervous Substance. 



Our knowledge of the chemical constitution of the nervous system is, in many regards, 

 quite unsatisfactory ; but these tissues contain certain elements that have been very satis- 

 factorily determined. The chemical characters of cholesterine, for example, have long been 

 known to physiologists, as well as the fact that this principle is a constant constituent of 

 the nervous substance, united in some way with the other proximate principles, so that 

 it does not appear in a crystalline form. Since we demonstrated, in 1862, the relations 

 of cholesterine to the processes of disassimilation, this principle has assumed its proper 

 place as one of the most important of the products of physiological waste of the organ- 

 ism. The origin and function of cholesterine, with the processes for its extraction from 

 the fluids and tissues of the body, have been fully considered under the head of excretion. 



