2i8 ELEMENTS OF SCIENCE 



Within the trunk is a cavity containing the heart, 

 lungs, kidneys, stomach, intestine, liver, &c. Within 

 the skull and backbone is a mass of white substance the 

 brain and spinal marrow. Delicate threads of this white 

 substance (nerves), and also tubes of various sizes (vessels), 

 traverse the body in all directions. The various " organs " 

 are, as before said, grouped in "i systems,"* and are 

 composed of "tissues." Thus, "fat" is adipose tissue, 

 flesh is muscular tissue, the outermost layer of the skin 

 is epithelial tissue, and its deeper layer is formed of 

 connective tissue. Bone is osseous tissue, the brain and 

 nerves are nervous tissue, gristle is cartilaginous tissue? 

 and the blood may be termed sanguineous tissue. 



The body is reducible to the ultimate chemical 

 elements above enumerated, t but before this extreme 

 reduction to its ultimate elements it can be shown to 

 consist of certain complex organic compounds or proxi- 

 mate elements, such as albumen (the substance of the 

 white of egg) and gelatine (the substance of jelly), and 

 others. 



The flesh consists of a multitude of delicate threads 

 called " muscular fibres," which are variously aggregated 

 in masses, and so form muscles. The heart is a four- 

 chambered muscular organ, the centre of two sets of tubes 

 arteries and veins the extremities of which are con- 

 nected by minute vessels termed capillaries. 



A third set of vessels lymphatics converge from all 

 parts of the body to two large veins. The arteries, 

 veins and heart, are full of blood, and the lymphatics of 

 an almost colourless fluid called lymph. These fluids 

 contain a multitude of minute bodies termed " blood 

 corpuscles." which are of two kinds, white and red. 



* See ante, p. 187. f See ante, p. 192. 



