656 THE BRAIN. 



the occipital lobe to 4.2 mm. at the dorsal summit of the precentral convolu- 

 tion, is, like other gray matter, composed of nerve-cells, and of nerve-fibres 

 and fibrils supported by neuroglia. The nerve-cells, at least the conspicuous 

 and easily recognized nerve-cells, are scattered, and appear, in sections, to be 

 imbedded in and separated from each other by a not inconsiderable but 

 variable quantity of somewhat peculiar ground substance, not unlike that 

 which forms so large a part of the molecular layer of the cerebellum. Part 

 of this ground substance, which apparently is not confined to any particular 

 layer, but stretches throughout the thickness of the cortex, is undoubtedly 

 neuroglial in nature, but part, and probably the greater part, is nervous in 

 nature ; it is largely composed of fine fibrils traversing it in various direc- 

 tions, the transverse sections of the fibrils giving it a characteristic dotted 

 or " molecular " appearance ; and the majority of these fine fibrils are prob- 

 ably the continuations of branching nerve-cells or dividing nerve-fibres, the 

 remainder being neuroglial fibrils. In this respect it resembles the molec- 

 ular layer of the cerebellum, but it is, to a much greater extent than is that 

 layer, traversed by medullated nerve-fibres, especially by fine medullated 

 fibres like those seen in the gray matter of the spinal cord ( 476). 



The nerve-cells imbedded in this ground substance in more or less dis- 

 tinct layers are of various kinds. The most conspicuous, abundant and cha- 

 racteristic nerve-cells found in the cortex of all regions of the cerebellum 

 are those which from their shape are called pyramidal cells. These vary 

 very much in size and have been distinguished as " small pyramidal " cells 

 averaging 12 p in length by 8 /j. in breadth, and " large pyramidal" cells, 

 sometimes called " ganglionic cells," of which the medium size is about 40 t>. 

 in length by 20 A* in. breadth. Some of the latter, occurring in special 

 regions, are of very large size, 120 /* by 50 fj., and have been called " giant 

 cells." 



The features of a " large pyramidal " cell are very characteristic. Such a 

 cell appears in a well-prepared vertical section of the cortex as an elongated 

 isosceles triangle placed vertically, with the base looking toward the under- 

 lying white substance and the tapering apex pointing to the surface. The 

 cell substance is finely granulated or fibrillated, the fibrilla3 sweeping round 

 in various directions ; it not unfrequently contains pigment. In the midst of 

 this cell substance rather near the base lies a large, clear, conspicuous round 

 or oval nucleolated nucleus. At the base the cell substance is prolonged 

 into a number of processes. One of these, generally starting from about the 

 middle of the base, runs for some distance without dividing, and soon acquir- 

 ing a medulla may be recognized as an axis-cylinder process ; the fibre to 

 which it gives origin sweeps with a more or less curved course into the sub- 

 jacent white matter. In some instances the axis-cylinder process, by a 

 T-division like that seen in a ganglion of a posterior root ( 93), gives rise 

 to two fibres, one of which may take a horizontal direction ; in some regions 

 of the cortex, the occipital for instance, the axis-cylinder process is said to 

 give rise by division to several fibres. The other processes from the base, 

 especially those from the angles of the triangle, rapidly branch into fine 

 fibrils which are soon lost to view in the ground substance. The apex of 

 the triangle is also prolonged into a process, which, giving off fine lateral 

 branches, makes, as it were, straight for the surface, but ultimately branch- 

 ing into fine fibrils is lost to view at some distance from the body of the cell. 

 The cell lies in a cavity of the ground substance, which it appears normally 

 to fill, but from the walls of which it sometimes shrinks, developing between 

 itself and the wall of the cavity a space which may contain not only lymph, 

 but occasionally leucocytes. In prepared specimens the retraction within its 

 cavity of the artificially shrunken cell may be often observed. 



