Nervous Tissuks. 21 



layers in connection with visceral organs or with the skin, and is thus 

 much less individual than the voluntary muscles in its relations to 

 particular parts. It forms the muscular portion (muscularis mucosae) 

 of the mucous tunic of the alimentary canal, and also a separate muscular 

 tunic lying in the outer portion of its wall (Fig. 15). In the muscular 

 tunic the fibres are arranged in both circular and longitudinal directions. 

 Involuntarv muscle also forms a small constituent of many organs, such 

 as certain glands, in which contractility is not a chief function. It forms 

 a large constituent of the wall of the urinogenital tubes, particularly the 

 bladder and the uterus. In association with elastic connective tissue it 

 is an important constituent of the walls of the bloodvessels. 



Although there is an underlying community of structure in the walls 

 of the bloodvessels, the two chief types of vessels, arteries and veins, 

 present conspicuous differences, both in functional behaviour, and in their 

 appearance in the dead animal. The differences are largely the result 

 of differences in the relative amounts 

 of the above-mentioned constituents. 

 The arteries are thick-walled, elastic 

 tubes, which, under the force of blood 

 from the heart, first become greatly 

 expanded, and then gradually con- 

 tract, so that the blood is forced into 

 the smaller capillary vessels. The 

 veins on the other hand are thinner- 

 walled, less elastic vessels, through 

 which the blood is forced largely 

 through the pressure from behind. 

 In the dead animal the arteries 

 appear white, flat or collapsed, and 



pmnfv TViPVpincnn fhp nfhpr Hand Fig. 16. Nerve-cell from the anterior grey 



enipt}.. ine vems on me otner nana column of the spinal cord (d. Fig. is): d., 



appear large and dark on account of dendrites; e.g., chromatophile granules; 



their distension with blood. 



4. Nervous Tissues. 



Nervous tissues form the basis of the central nervous system and 

 of the outlying nerves and ganglia. They comprise two kinds of elements 

 — nerve cells and nerve fibres. In the central nervous system these 

 elements are imbedded in a mass of neutral tissue, the neuroglia. 



Nerve cells are characteristic of the central nervous system and of 

 the spinal and sympathetic ganglia. They differ greatly in form, but 

 tvpically each consists of a cell-body (Fig. 16) bearing two kinds of 

 processes — a fibre-process, the neuraxis or neurite, and a series of branched 

 protoplasmic processes, the dendrites. The cell-body is distinguished by 

 the presence in its interior of granular masses, the chromatophile or 

 tigroid bodies. The latter extend into the dendrites, but not into the 

 neuraxis. The dendrites may be greatly elaborated, and may be present 

 to a considerable number. The neuraxis is a nerve fibre process. Since 

 it continues as the central portion or axial cord of a nerve fibre, it may 

 traverse a relatively enormous distance on its way to a peripheral organ. 



