CELLULAR DIFFERENTIATION 63 



the medullary sheath, consisting of a -framework filled with a 

 fatty material, surrounded by (2) Schwann's sheath, a homoge- 

 neous sheath with occasional nuclei along its course (Fig. 30, Z?). 

 Fibres possessing the medullary sheath are called meduUated 

 or white fibres. 



A nerve cell together with its processes is called a neuron. 

 The whole nervous system may be considered as made up of such 

 units, which connect with each other by the delicate terminal 

 branches of the outgrowths. See Fig. 36. 



85. Origin of Nervous Tissue. — Nervous tissue always arises from the ectoderm 

 of the embryo, so far as we know. In some of the lower forms of animals, as the 

 Coelenterata, the nervous cells may be derived individually from the epithelium. 

 In such instances they have a close connection with those muscle elements which 

 are also of epithelial origin (see § 82). In the higher forms the origin of the nervous 

 matter from the ectoderm is somewhat less direct but essentially similar. The 

 connection of the nervous centres with the muscles and glands, etc., in the higher 

 animals is a secondary condition and is the result of the growth of the nerve fibres 

 from the centres toward such organs. What directs their growth to the right place 

 when the fibres begin to grow, we do not know. 



86. Summary. 



1. The individual becomes complex by the increase of the 

 number of cells, and by their differentiation. 



2. A tissue consists of a group of similar cells with their 

 products, which are adapted to the performance of special work 

 or function. 



3. Tissues differ morphologically in respect to the form, ar- 

 rangement, and structure of the cells, and in the amount, 

 arrangement and consistency of the intercellular substance. 



4. Physiological differentiation accompanies the morpho- 

 logical, the division of labor becoming very complete in the 

 higher forms. The physiological value of a tissue may depend 

 either upon the cells or the intercellular substance. 



5. Tissues may be classified as follows: 

 A. The vegetative or passive tissues. 



I. EpitheUal: — 



function: — protection, absorption, secretion, 



sensation, reproduction, etc. . 

 kind: — pavement, columnar, ciliate, glandular, 



sensory, muscular, reproductive, etc. 



