312 THE CHEMISTRY OF ANIMAL PHYSIOLOGY 



cartilage, and such substances. Collagen and elastin are 

 two proteins contained in connective tissue. Gelatine can 

 be prepared from collagen by boiling it in water. 



Fig. 77. — Adipose tissue, highly magnified, a, star-like appearance, from 



crystallization of fatty acids. (Gray.) 



231. Blood. — This liquid serves to carry nutrient material 

 to all parts of the body, supplying the various tissues with 

 what they need for growth and repair. It also serves to 

 carry away the waste products of metabolic activity. The 

 blood may be compared roughly to the sap of plants in so 

 far as it supplies soluble food material to various parts of the 

 living organism. The sap of plants, it will be remembered 

 (Section 16), is forced through tracheae up the stem and 

 into the leaves by osmotic pressure or surface tension, or a 

 combination of both; whereas the blood is forced through 

 a system of open canals or vessels to all parts of the animal 

 body by the pressure of a pump which is called the heart. 

 A rhythmic expansion and contraction of muscles around the 

 heart pull the blood in and force it out, thus keeping up a 

 continuous circulation of blood through the vessels. The 

 main channels which bring blood to the heart are called veins, 

 while those carrying blood away from the heart are called 



