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 (myofibrillae), 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 myofibrillae, 

 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- 



