CONNECTIVE TISSUE 311 



(Fig. 76). It causes motion in the animal, having the 

 power of contracting and expanding when stimulated 

 by the nerves. This contraction may be transmitted to 

 the bones and cause locomotion, or may be merely a rhyth- 

 mical contraction and expansion, causing the well-known 

 movements of the heart, lungs, and other organs. The 

 muscle substance is composed largely of proteins, but also 

 some glycogen, dextrose, potassium phosphate, and nitrog- 

 enous extractives. 1 It is about 75 per cent, water and 

 25 per cent, solids. The principal protein is myosinogen, 

 liquid in living muscle, but changing to solid myosin in 

 dead muscle. Living muscle at rest is alkaline in character; 

 active or dead muscle is slightly acid due to the formation 

 of an isomer of lactic acid called sarcolactic acid. Muscles are 

 bathed in lymph and permeated with blood-vessels. When 

 a muscle does work, the dextrose is oxidized by the oxygen 

 brought to it in the blood stream, and carbon dioxide is 

 given off. Heat is also developed by this oxidation. 



228. Fatty Tissue. — Besides the fats or fixed oils which 

 constitute a part of all protoplasmic material, there are in 

 animals various deposits of fat in the muscles, bone marrow, 

 liver, and so-called adipose tissue (Fig. 77). The latter is a 

 mass of cells each composed mainly of a large globule of 

 fat. This adipose tissue usually lies just under the skin. 

 Chemically fat is composed for the most part of glycerides 

 of stearic, palmitic, and oleic acids, as in the case of plants 

 (Section 63, et seq.). 



229. Epithelial Tissue. — Epithelial tissue lines all the 

 surfaces of the body — the skin on the outside and the 

 mucous membrane on the inside, such as the lining of the 

 alimentary canal and body cavities. Such epithelial cells 

 as the hair, nails, hoofs, etc., are composed largely of a 

 protein called keratin which is rich in sulphur. Epithelial 

 cells of the mucous membrane are largely mucin which gives 

 this tissue its viscid character. 



230. Connective Tissue. — This material serves to bind 

 together the various body parts, and composes the tendons, 



1 Nitrogen compounds, not proteins, soluble in water. 



