54 THE CONTRACTILE TISSUES. 



blood, and in so doing keep up an average quantity. In starvation the 

 quantity (and -quality) of the blood is maintained for a long time at the 

 expense of the tissues, so that after some days' privation of food and drink, 

 while the fat, the muscles, and other tissues have been largely diminished, 

 the quantity of blood remains nearly the same. 



The total quantity of blood present in an animal body is estimated in the fol- 

 lowing way : As much blood as possible is allowed to escape from the vessels ; 

 this is measured directly. The vessels are then washed out with water or normal 

 saline solution, and the washings carefully collected, mixed, and measured. A 

 known quantity of blood is diluted with water or normal saline solution until it 

 possesses the same tint as a measured specimen of the washings. This gives the 

 amount of blood (or rather of haemoglobin) in the measured specimen, from which 

 the total quantity in the whole washings is calculated. Lastly, the whole body is 

 carefully minced and washed free from blood. The washings are collected and 

 filtered, and the amount of blood in them is estimated, as before, by comparison 

 with a specimen of diluted blood. The quantity of blood, as calculated from the 

 two washings, together with the escaped and directly measured blood, gives the 

 total quantity of blood in the body. 



The method is not free from objections, but other methods are even more 

 imperfect. 



The blood is in round numbers distributed as follows : 



About one-fourth in the heart, lungs, large arteries and veins. 



About one-fourth in the liver. 



About one-fourth in the skeletal muscles. 



About one-fourth in the other organs. 



Since in the heart and great bloodvessels the blood is simply in transit, 

 without undergoing any great changes (and in the lungs, as far as we know, 

 being limited to respiratory changes), it follows that the alterations which 

 take place in the blood passing through the liver and skeletal muscles far 

 exceed those which occur in the rest of the body. 



CHAPTER II. 



THE CONTRACTILE TISSUES. 



39. IN order that the blood may nourish the several tissues it is carried 

 to and from them by the vascular mechanism ; and this carriage entails 

 active movements. In order that the blood may adequately nourish the tis- 

 sues, it must be replenished by food from the alimentary canal, and purified 

 from waste by the excretory organs ; and both these processes entail move- 

 ments. Hence before we proceed further we must study some of the general 

 characters of the movements of the body. 



Most of the movements of the body are carried out by means of the mus- 

 cles of the trunk and limbs, which being connected with the skeleton are 

 frequently called skeletal muscles. A skeletal muscle when subjected to 

 certain influences suddenly shortens, bringing its two ends nearer together; 

 and it is the shortening, acting upon various bony levers or by help of other 

 mechanical arrangements, which produces the movement. Such a temporary 

 shortening, called forth by certain influences, and due as we shall see to 

 changes taking place in the muscular tissue forming the chief part of the 



