20 INTRODUCTION. 
apparently acts as an insulator, preventing nervous impulses from pass- 
ing from one axon to another. This sheath does not continue over the 
dendrites. Frequently the dendrites of two neurons interlace for the 
transference of stimuli from one to the other, but the present opinion is 
that, at least in vertebrates, there is no actual continuity of substance 
between neurons, only an interlacing of terminal twigs. The medullary 
sheath is not cellular, but frequently fibres may be surrounded by a 
sheath of Schwann (s), with scattered nuclei. This has been re- 
garded as mesenchymatous, but recent researches tend to show that 
it is ectodermal, its cells coming from the nervous system. 
Nervous tissue consists of these neurons plus connective tissue and 
glia cells. A nerve, as found in dissection, consists of numbers of 
axons, bound together by a connective-tissue envelope (perineureum). 
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 gray matter is 
aggregated in the course of a nerve, it causes an enlargement 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 muscular tissue. 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 on their interior a 
large amount of contractile substance (myofibrille), which on stimula- 
tion, contracts, shortening the cell. In the vertebrates, muscular tissue 
always arises from the mesoderm, yet two types are recognized, differing 
markedly in origin, appearance and physiological action. 
The smooth or involuntary muscles arise from the mesenchyme. 
They consist of long and spindle-shaped cells (fig. 13, A), each with a 
single nucleus, the protoplasm traversed by numerous myofibrille, 
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 voluntary muscular tis- 
