122 CHAPTER 13. 



set, which are partly dependent on the will and partly involuntary, 

 called Mixed muscles. Such are the diaphragm, pharynx and sphincters. 



Nerves are abundant in muscular fibres, and so also is the supply of 

 blood. It courses freely through the blood-vessels which lie parallel to 

 the fibres and through their capillaries. The red colour of muscle is due 

 to the blood in it. The growth of muscle is due to increase in the size, 

 but not in the number of the cells of the fibres. 



The function of muscle is its power of contractility. Its contractions 

 are the means by which the various parts of the frame are moved on each 

 other. The entire of a muscle does not move at once, but successive por- 

 tions contract, whilst the other fibres are at rest. The stimulus to con- 

 traction is the impression made on a sensitive nerve, the influence of 

 which is conveyed to a motor nerve. Muscles during contraction become 

 harder, shorter and broader, but do not undergo any change in bulk. 

 What they lose in length, they gain in breadth. 



Muscles are connected with bones by means of tendons ; or in other 

 words the end of each muscle before its insertion becomes tendinous, and 

 this tendinous end is inserted into a bone. The more fixed end of a 

 muscle is called its origin, and the more moveable its insertion. 



Under the influence of strong work combined with good feeding and 

 good health muscular tissue increases in size and hardness. In other 

 words the flesh becomes full and hard. With good feeding without work 

 muscular tissue, though it may increase in size, becomes soft and flabby, 

 being in fact much infiltrated with fat. With bad feeding, especially 

 when combined with hard work, muscle loses its development, and be- 

 comes wasted and pale, and its power of contraction is greatly diminished. 



238. Adipose tissue or Fat. 



Adipose tissue or Fat consists of a network of very minute cells, in 

 which is enclosed the oily or fatty matter. The walls of these cells, which 

 are composed of areolar tissue, are exceedingly thin. Capillary vessels, 

 from which the fatty matter is secreted, are looped round each cell. 



Adipose tissue is found in almost every part of the body. It is 

 mechanically useful in padding exposed parts. Its particles, though they 

 do not move about among themselves, as has often been supposed, yet 

 yield very readily to pressure ; and on this account fat when placed as a 

 cushion under any organ is very useful in obviating the effect of pressure 

 on it. It forms for instance a cushion for the eye and for many joints. 

 It lessens the brittleness of .bones, and also assists in retaining the heat 

 within the body. When an animal is underfed, the system appropriates 

 to its own use the fatty material contained in the cells. Hence under 

 such circumstances a fat horse soon becomes thin. 



239. Pigments. 



The coloring matter of the skin, hair, blood, bile or any other consti- 

 tuent of the body is termed its pigment. 



