I50 



COMPARATIVE ANATOMY 



tissue, the perineurium (Fig. 112), extensions of which (endoneurium) 

 may penetrate into the bundle. Larger nerves consist of several or many 

 bundles all tied together by connective tissue and enwrapped by a rela- 

 tively thick epineurium. Small blood vessels traverse the connective- 

 tissue layers of the nerve. 



The component neuraxons of a nerve may include one or more of the 

 four types of conductor, somatic afferent and efferent and visceral afferent 

 and efferent. All four kinds of "nerve components" occur in a spinal 

 nerve. In cranial nerves fibers which are connected with the special 

 sensory mechanisms of the head are designated as "special" components. 



Fat cells. ;:r 



Artery. 



Bundles of nerve fibers 



Epineurium. 



Perineurium. 



Endoneurium. 



Fig. 112. — Structure of a nerve. The figure represents a small part of a transverse 

 section of a large nerve constituted of many bundles of medullated fibers. X20. 

 (From Bremer, Text-book of Histology.) 



Tissues Serving for Mechanical Support 



Protoplasm is a substance of semi-fluid or gelatinous consistency. 

 An elephant constituted of protoplasm only would be a mechanical 

 impossibility. Large animals, especially if they are land animals, require 

 mechanical support. That essentially protoplasmic and socially organized 

 being which is mankind provides for certain of its physical needs, both 

 individual and collective, by use of various mechanical, architectural and 

 engineering contrivances which are external to the human body and con- 

 structed of non-hving materials derived from the environment. Similarly 

 the organized cellular protoplasm which is an animal appropriates various 

 materials from the environment and builds them into non-hving structures 

 which are external to the cells and physically adapted to the mechanical 

 needs of the animal as a whole and of its parts. Just as man uses steel 

 wires, various cements and masonries, so the animal has its connective- 

 tissue fibers, intercellular cements and that most delicate masonry, bone. 



