32 ANATOMY OF THE RABBIT 



constituents of the central nervous organs produce characteristic 

 patterns according to their varying concentration. Where cell 

 bodies, dendrites, and unmyelinated fibres preponderate, the tissue 

 has a greyish colour and is hence distinguished as grey matter; 

 while the concentration of myelinated fibres produces an opaque 

 white appearance similar to that seen in the larger peripheral 

 nerves, whence the tissue is described as white matter. In the 

 spinal cord (Fig. 18) the grey matter is disposed as a central core, 

 the white substance as a peripheral investment. An essentially 

 similar though much elaborated distribution is found in the basal 

 portion of the brain (Fig. 123), but the characteristic pattern in 

 the cerebral hemispheres and in the cerebellum is one in which the 

 grey substance forms a peripheral, investing, or cortical layer (Figs. 

 115, 117, 123). 



In order that the conduction of excitation waves through the 

 nerve cells and fibres may be effective, the neurons must be linked 

 up in functionally useful patterns, the complexity of which is often 

 almost inconceivably great. Each neuron receives or transmits 

 impulses from or to others at special points of intimate contact, 

 the synapses, through which the excitation can pass in only one 

 direction though within a single neuron it can be propagated in all 

 directions. The synapses occur almost entirely within the grey 

 matter. 



Neuroglia is a special type of connective tissue unrelated de- 

 velopmentally to the true connective tissues but having a common 

 embryonic origin with the nerve cells. The neuroglia cells are 

 much branched but lack the distinctive features of neurons. Among 

 the true neuroglia elements occur small, phagocytic, migratory cells 

 related in origin to ordinary connective tissue. These are known 

 as microglia. 



5. Blood and Lymph 



Blood is fundamentally a cellular material, but owing to the 

 fact that the intercellular matrix takes the form of a liquid medium, 

 the plasma, in which the cells, or corpuscles, are suspended, its 

 features largely differ from those of the ordinary tissues of the body. 

 On account of its liquid character, the appearances presented by 

 blood in dissection, especially of preserved animals, are almost 



