NERVE-TISSUE. 283 



partly from a columnar formation, closely connected with the nucleus of the 

 vagus and traceable into the anterior column of the spinal cord. 



The hypoglossal nerve has its nucleus in the posterior portion of the fossa 

 rhomboidalis, close to the median line. There is also a direct passage of 

 fibers from the crus cerebri into the hypoglossus by way of the raphe. 



(B) Gray Substance. The gray substance is the only nerve- 

 tissue found in the brain of the lower vertebrates. Here, instead 

 of ganglionic elements, nuclei are present, which, especially 

 around the ventricles, collect in regular rows, representing in its 

 simplest relations the bioplasson of the nervous center. (See 

 Fig. 118.) 



The presence of connective tissue in the gray substance is 

 unquestionable, as it is the carrier of the numerous blood-vessels 

 of the gray substance. The finest ramifications of connective 

 tissue, however, have not been discovered, but are still the sub- 

 ject of animated controversy among histologists. It seems that 

 the finest offshoots of the ganglionic elements, producing an 

 extremely delicate reticulum, first discovered by T. Gerlacli, 

 deserve to be classified among nervous structures, inasmuch as 

 in this reticulum there is no indication of a basis-substance an 

 essential part of all varieties of connective tissue. It may be that 

 connective and nervous tissue blend with each other so intimately 

 that an accurate determination of either of them is impossible. 



The blood-vessels of the gray substance are characterized by 

 the presence of a lymph-sheath. His was the first to draw atten- 

 tion to this fact. According to him, each blood-vessel is en- 

 sheathed by an adventitial coat of endothelial structure, and the 

 space between the tube of the blood-vessel and the investing tube 

 of the lymph-vessel varies greatly in width. Boll's view con- 

 cerning the lymph-sheath is somewhat different. 



The cortex of the cerebellum is composed of three layers. The 

 outermost is called the gray layer, and exhibits, with low powers 

 of the microscope, a delicate granular appearance, which, with 

 high powers, proves to be a reticulum, considered by histolo- 

 gists to be a connective-tissue formation. Within the reticulum 

 there are scanty, small, branching ganglionic elements 5 on the 

 innermost portion we notice fibrous tracts, in a direction par- 

 allel to the surface. The middle so-called cell-layer contains 

 large, branching, and nucleated ganglionic elements, mostly pear- 

 shaped, standing in a vertical or oblique direction to the surface. 

 These bodies, in honor of their discoverer, are termed Purkinje's 

 cells. From the outer pole of each corpuscle originates an offshoot, 



