36 THE HUMAN BODY. 



readily transmit molecular disturbances. When its equilib- 

 rium is upset at one end, a nerve-fibre transmits to its other 

 end a molecular movement known as a "nervous impulse" 

 and so can excite parts distant from the original exciting 

 force. Nerve-fibres place, on the one hand, the irritable 

 tissues in connection with the automatic, co-ordinating, and 

 sensory; and on the other put the three latter in communica- 

 tion with the muscular, secretory and other tissues. 



9. PROTECTIVE TISSUES. These consist of certain cells lin- 

 ing cavities inside the body and called epithelial cells, and cells 

 covering the whole exterior of the Body and forming epider- 

 mis, hairs and nails. The enamel which 

 covers the teeth belongs also to this 

 group. 



The class of protective tissues is, how- 

 ever, even, more artificial than that of the 

 nutritive tissues, and cannot be defined by 

 positive characters. Many epithelial cells 

 are secretory, excretory or receptive; and 

 ciliated cells have already been included 

 \r^ FI -th amori g the motor tissues, although from 

 lium-ceiis from the sur- the fact that the movements of their cilia 



face of the peritoneum. , , _, _ . _, 



, cell-body ; c, nucleus; so on in separated cells and independently 



6, nucleoli. & . ui i *.- v ^ 



of recognizable external stimuli, they 

 might well have been put among the automatic. The protec- 

 tive tissues may be best defined as including cells which cover 

 free surfaces, and whose functions are mainly mechanical or 

 physical. In their simplest form epithelial cells are flat 

 scales, as, for example, those represented in Fig. HB, from 

 the lining membrane of the abdominal cavity. 



10. THE EEPRODUCTIVE TISSUES. These are concerned in 

 the production of new individuals, and in the Human Body 

 are of two kinds, located in different sexes. The conjunction 

 of the products of each sex is necessary for the origination 

 of offspring, since the female product, egg-cell or ovum, 

 which directly develops into the new human being, remains 

 dormant until it has \>QZ\\ fertilized, and fertilization consists 

 essentially in the fusion of its nucleus with the nucleus of a 

 cell produced by the male. 



The Combination of Tissues to Form Organs. The va- 

 rious tissues above enumerated forming the building materials 

 of the Body, anatomy is primarily concerned with their struc- 



