NERVOUS TISSUKS 29 



of its wall (Fig. 15). In the muscular tunic the fibres are arranged 

 in both circular and longitudinal directions. Involuntary muscle 

 also forms a small constituent 

 of many organs, such as cer- \* 

 tain glands, in which contractil- \ 



ity is not a chief function. It "^ ^ 4- 



is a large constituent of the wall "^ . c.q. 



of the urinogenital tubes, par- l^>i! ^. . 



ticularly the bladder and the -^v*?-4i.- 



uterus. In association with elas- 

 tic connective tissue, it is an im- 

 portant constituent of the walls ^""^ ^ 

 of the blood-vessels. ^.r. 



The fundamental cause of '•"■- l^'- Ncrvccell from the ventral grey 



. column of the spinal cord (cf. Fig. 18): d., 



the shortenmg of muscle fibres dcntrltes; e.g., chromotophile granules; nr., 



ncurite. 



is not yet understood nor is 



the significance of the transverse striations, when these are {pres- 

 ent, known. 



4. Nervous Tissues 



Nervous tissues are the essential components of the central 

 nervous system and of the outlying nerves and ganglia. They 

 comprise two kinds of elements — neurons or nerve cells and 

 neuroglia cells. The former alone carry on the essential nervous 

 functions, while the latter are supporting structures forming in the 

 central nervous organs a mass of neutral tissue in which the neurons 

 are imbedded. 



Nerve cells differ greatly in form, but typically each consists of 

 a cell-body (Fig. Ki) bearing two kinds of processes — a single 

 axon, neurite, or neuraxis, and a series of branched protoplasmic 

 processes, the dendrites. The nerve-cell body is characterized 

 by the presence in its cytoplasm of granular masses, the chro- 

 matophile or tigroid bodies, or Nissl granules, the size and arrange- 

 ment of which are distinctive of certain types of nerve cells. These 

 extend into the dendrites but not into the axon. The dendrites, 

 which may be greatly elaborated and may be few or many, conduct 

 nerve impulses towards the cell body and the axon conducts them 

 away from it. The latter may traverse a relatively enormous dis- 



