STRUCTURE OF THE HUMAN BODY 49 



internal organs, as has already been seen in the case of the 

 heart. It also enters into the walls of the blood-vessels, 

 serving to regulate their size, and the walls of the food-tube 

 are also largely composed of the same substance, which here 

 exerts a kind of squeezing action by which the digesting food 

 is gradually passed on. The muscle, however, making up the 

 flesh of the body is different in structure from that found in 

 internal organs, and is termed voluntary, being under the direct 

 control of the will; while the muscle of the heart, &c., is in- 

 voluntary, because it cannot be regulated in this way. 



2. CILIARY ACTION. Movement in the human body is, how- 

 ever, not all due to muscular action, but can be brought about 

 in two other ways. One of these is ciliary action, of which a 

 good example is afforded by the lining of the windpipe. If this 

 is examined under a compound microscope it will be found to 

 be lined with fragments of protoplasm (cells), covered thickly 

 with short protoplasmic threads known as cilia (fig. 25). Each 

 cilium is able to alternately bend and straighten 

 itself, and when numerous cilia work together, as is 

 usually the case, they are able to move along small 

 particles placed upon the surface they beset. Their 

 action in the case mentioned is to sweep particles Fi g .2 5; -cenTofciii- 

 of dust, &c., towards the exterior, thus keeping a ^ d g j^ um ' much 

 the ciliated surface clean. The last kind of move- 

 ment is the peculiar creeping motion exemplified by the colourless 

 corpuscles of blood and lymph (fig. 15). It is said to be amoeboid. 



The two kinds of movement just described are much more 

 primitive than muscular action, and, as might be expected, 

 play a much more leading part in the lower forms of life than 

 they do in such a complicated organism as a human being. 



NERVOUS SYSTEM AND SENSE ORGANS. 



The great complexity of the body, with its numerous subtly 

 interwoven functions, demands some means of controlling and 

 correlating these, and of keeping the body as a whole in touch 

 with the outer world. The means is found , in those organs to 

 which the terms nervous system and sense organs are applied. 



THE NERVOUS SYSTEM. The nervous system essentially 

 consists of certain central organs, which may be regarded as 



VOL. I. 



