HISTOLOGY 25 



axons, bound together by a connective-tissue envelope (perineu- 

 reum) . The myelin gives these nerves a white color. In the brain 

 and spinal cord there are tracts of medullated fibres (white matter) 

 while the parts with abundant nerve cells are gray. When such 

 nerve cells are aggregated in the course of a nerve, it causes an en- 

 largement called a ganglion. Interlacing among the neurons in 

 brain and spinal cord is the neuroglia, which is also derived from the 

 ectoderm, and acts as a support, but has no nervous functions. 

 Certain of these glia cells develop many branches (mossy cells) which 

 twine among nerve cells, axons, and dendrites. 



Muscular Tissues 



While several kinds of cells have the power of changing shape, 

 those composing the muscular tissues possess it in a marked degree, 

 acting quickly and with force, so that these tissues are preeminently 

 the tissues of motion. The cells become elongate and develop in 

 their interior a large amount of contractile substance (myofibriilae) , 

 which on stimulation, contracts, shortening the cell. In the verte- 

 brates, muscular tissue always arises from the mesoderm. Two 

 types are recognized, differing markedly in origin, appearance and 

 physiological action. 



The smooth or involimtary muscles arise from the mesenchyme. 

 They consist of long and spindle-shaped cells (fig. 16, A), each with a 

 single nucleus, the protoplasm traversed by numerous myofibriilae, 

 which appear like fine longitudinal lines. In the vertebrates the 

 smooth muscle is not under control of the will; it contracts slowly. 



In contrast to the smooth is the striped or volimtary muscular 

 tissue, which arises from a modification of the mesothelium. Except 

 in the case of the muscles of the heart, the striped tissue is under con- 

 trol of the will; it usually occurs in larger masses than does the smooth, 

 and is capable of rapid contraction. It differs structurally from 

 smooth muscle. Instead of distinct, uninucleate cells there are long 

 cylindrical elements (fig. 16, B), the primitive fibres, each with 

 several nuclei in the interior in lower vertebrates, on the periphery in 

 the higher. Most of the protoplasm of the fibre has been altered to 

 minute contractile fibrillac, each crossed by lighter and darker bands, 

 and as these correspond in position in the different fibrillar, they give 

 the fibre its characteristic cross-banded appearance. 



The primitive fibres rarely branch at their extremities. Each is 



