50 ZOOLOGY. 



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

 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. 34. 



82. 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 79). In the higher forms the origin of the nervous mat- 

 ter 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 toward such organs. What directs 

 their growth to the right place when the fibres begin to grow, we do 

 not know. 



83. 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, 

 arrangement, 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 de- 

 pend either upon the cells or the intercellular substance. 



5. Tissues may be classified as follows : 

 A. The vegetative or passive tissues. 



I. Epithelial : 



function : protection, absorption, secretion, 



sensation, reproduction, etc. 

 kind : pavement, columnar, ciliate, glandular, 



sensory, muscular, reproductive, etc. 



II. Supportive or connective: 



function : binding, support, protection. 



