GENERAL HISTOLOGY: CELLS AND TISSUES 31 



with granules, which are fine in the common type of leucocyte, coarse in a rarer 

 type. 



(&) Large lymphocytes : These are nearly spherical cells, with clear cytoplasm 

 and a small rounded nucleus. 



(c) Small lymphocytes : They are smaller than the preceding with relatively 

 large nuclei, covered by a small rim of cytoplasm. 



5. Nervous tissue. Nervous tissue is composed of nerve cells, and the struc- 

 tures which support them (Holmes, pp. 131-33). A nerve cell consists of a 

 central portion, called the cell body which contains a large nucleus, and of slender 

 processes which extend out from this cell body often to long distances. Portions 

 of nervous tissue which consist almost entirely of cell bodies are designated as 

 gray matter, while those which consist of the processes constitute the white matter. 

 The processes of a nerve cell are of two kinds : those that convey the impulses into 

 the cell, called dendrites, usually very numerous and much branched, and that 

 one which conveys the impulse away from the cell, always single and unbranched 

 or slightly branched, named the axone. What are called nerves are bundles of 

 axones. 



a) Brain cells: In order to demonstrate the entire nerve cell with its processes 

 it is necessary to cut thick slices of the brain, and to devise a special staining 

 method since the processes do not take ordinary stains. A method was originated 

 by an Italian histologist named Golgi, by which the entire cell is blackened by a 

 deposit of silver upon it. Examine a slide of brain provided, stained by the 

 Golgi method, either brain cells of the rabbit (Lepus), or human cerebellum. 

 Use low power only, as the sections are thick. Note the numerous black branched 

 objects upon the slide. Each is a nerve cell with its processes. Pick out a 

 favorable place on the slide (A) and study one of the cells. Each consists of a 

 rounded or triangular cell body from which spring several processes. In the 

 case of the rabbit, one large, stout process springs from the pointed end of the 

 cell body and several branched processes from the rounded end. In the cells 

 of the cerebellum, two or three stout processes (dendrites) spring from one side 

 of the cell and immediately break up 'into an exceedingly complicated system 

 of branches, while from the opposite surface of the cell the small slender axone 

 arises (it is not always visible). Draw a brain cell. 



b) Motor cells of the spinal cord of the frog: In order to demonstrate the 

 actual structure of the cell body, the usual thin sections, stained in the ordinary 

 way, are employed. Examine the slide "Spinal cord frog" with your lowest 

 power. Identify the central large oval body on the slide (all other objects are 

 to be disregarded). In this oval object, which is the cross-section of the spinal 

 cord, observe two general regions, a central, denser, slightly darker region, the 

 gray matter, trapezoidal in shape and containing numerous darkly stained cell 

 bodies; and an outer region, lighter, more open, and with only a few small 

 cell bodies, the white matter. In the two corners of the lower base of the trapezoid 



