520 NERVOUS SYSTEM. 



ably the only structures directly concerned in innervation, are the following 

 accessory anatomical elements : 1, outer coverings surrounding some of the 

 cells ; 2, intercellular, granular matter ; 3, peculiar corpuscles, called myelo- 

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



Certain of the cells in the spinal ganglia and in the ganglia of the sym- 

 pathetic system are surrounded with a 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 is composed of a layer of very delicate endothelium. 

 Its physiological significance is not apparent. 



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

 stance between the cells, which closely resembles the granular contents of 

 the cells themselves. In addition to this granular matter, Robin has de- 

 scribed peculiar anatomical elements which he 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 be 

 ing by far the more abundant. The nuclei are rounded or ovoid, witl 

 strongly accentuated borders, are unaffected by acetic acid, finely granuls 

 and generally without nucleoli. The cells are rounded or slightly poly- 

 hedric, pale, clear or very slightly granular, and contain bodies similar to the 

 free nuclei. The free nuclei are -g-gVo to 40 * 00 of an inch (5 to 6 ft) in diam- 

 eter, and the cells measure ^^ 7 to 20*001 an( ^ sometimes y^Vir of an inch (10, 

 12 and 15 /*,). These elements also exist in the second layer of the retina. 



In the cerebro-spinal centres there is a delicate stroma of connective 

 sue, chiefly in the form of stellate, branching cells, which serves in a mes 

 ure, to support the nervous elements. This tissue, which is peculiar to 

 white substance of the encephalon and spinal cord, is called neuroglia. 



The blood-vessels of the nerve-centres form a capillary net- work wit 

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



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

 has already been described in connection with the anatomy of the lymphatic 

 system. The blood-vessels here are surrounded by what have been called 

 perivascular canals, first described by Eobin and afterward shown by His 

 and Eobin to be radicles of the lymphatic system. 



Composition of the Nervous Substance. The chemistry of the nervous 

 substance, as far as it is understood, throws little light on its physiology. 

 Certain albuminoids have been extracted which do not possess more than a 

 purely chemical interest. The substance called cerebrine is composed of 

 carbon, hydrogen, oxygen and nitrogen, without either sulphur or phosphorus. 

 Protagon is a nitrogenized substance containing phosphorus (Liebreich, 1865). 

 By some chemists protagon is thought to be a mixture of cerebrine and 

 lecethine. Lecethine is regarded as a nitrogenous fat. Other substances 

 which have been extracted xanthine, hypoxanthine, inosite, creatine and 

 various volatile fatty acids have no special physiological interest connected 

 with the nervous system and are found in many other situations. Cholester- 



