130 HISTOLOGY 



side with a ground membrane Z (Fig. 122), and states that they are some- 

 what narrower than a sarcomere (i.e., the distance between two successive 

 ground membranes) . He regards them as the places where new sarcomeres 

 form, thus providing for the growth of the heart. Jordan and Steele, 

 among others, consider that they are places where individual fibrils are 

 contracted, and the fact that they are shorter than adjacent sarcomeres 

 favors this interpretation. The discs may extend straight across the fiber, 

 but frequently they are broken into "steps" as shown in the figure. 



There are, therefore, three peculiarities of cardiac muscle through which 

 it differs conspicuously from skeletal muscle, namely, its anastomosing 

 fibers, central nuclei, and intercalated discs. 



NERVOUS TISSUE. 



General features. In nervous tissue the protoplasmic functions of irri- 

 tability and conductivity attain their highest development. Irritability 

 is that property which enables the cell to react to various stimuli, such as 

 pressure or light; and through conductivity the effects of stimulation are 

 transmitted to distant parts of the cell, or to adjacent cells. In all animals 

 the cells of the outer or ectodermal layer are those most exposed to stimu- 

 lation, and the ectoderm accordingly gives rise to the entire nervous system. 

 In some animals all the ectodermal cells have been described as equally 

 responsive to stimulation, and the name "sensory layer" has been applied 

 to the ectoderm as a whole. Usually, however, the sensory cells become 

 specialized in definite and limited areas of the ectoderm. M. Schultze 

 (1862) showed that the sensory cells of the nose and eye are epithelial 

 elements, the bases of which are prolonged into filaments which serve as 

 nerves to convey sensation. He taught that the specific functions of the 

 sense organs depend on their respective epithelial cells, which accordingly 

 may be designated as olfactory, gustatory, auditory or visual cells. 



Not only does the ectoderm produce sensory neuro-epithelial cells, the 

 nucleated bodies of which remain in the epithelium, but it gives rise to more 

 deeply placed nerve cells, which connect with the epithelial cells and place 

 them in communication with the muscles. In simple forms of animals this 

 connection is very direct, and the response of the muscle to epithelial 

 stimulation is quite automatic. In the higher animals there are both 

 direct and indirect paths from the sensory endings to the muscles, and mus- 

 cular action may be inhibited or initiated by certain of the centrally 

 placed nerve cells. 



The centrally placed cells in vertebrates constitute the spinal cord and 

 brain, which together form the central nervous system. The bundles of 

 fibers which convey impulses to and from the central nervous system, to- 

 gether with the cells associated with them, constitute the peripheral nervous 

 system. 



