268 THE BLOOD. 



Involuntary and Unconscious Discharges. If by reason of 

 disease or of injury the middle or the upper portions of the cord 

 become so disorganized as to cut off communication with the 

 brain, while at the same time the lower portion is in normal con- 

 dition, the act of defecation takes place when the rectum becomes 

 sufficiently distended to stimulate the anospinal center to action ; 

 but there is no power to retard nor is there any consciousness of 

 it, since the connection with the brain is severed. Under these 

 circumstances the discharges are involuntary and unconscious. 

 If the lumbar portion of the cord is the seat of injury or of 

 disease to such an extent as to destroy this center, the sphincter is 

 permanently relaxed, and the feces are discharged as fast as they 

 reach the anus. 



THE BLOOD. 



The office of the blood is twofold : 1. It carries to the tissues 

 of the body the materials which they need for their nourishment, 

 and, in the case of glands, for their secretion; and 2. It takes 

 from the tissues the materials which result from their destructive 

 metabolism waste materials which it carries to those organs 

 whose function it is to eliminate them, as, for instance, urea to the 

 kidneys. The blood may be likened to a river which bears to the 

 inhabitants along its banks their daily food, and into which at the 

 same time their waste is discharged and carried to the sea. 



Physical Properties of Blood. Blood is in general red in 

 color and alkaline in reaction when tested with litmus-paper, and 

 has in man a specific gravity of about ] 060, although this varies 

 in men, women, and children, being less in the last, except at 

 birth, when it is 1066. The specific gravity of the corpuscles is 

 greater than that of the plasma. 



Method of Obtaining the Specific Gravity of Blood. The most 

 convenient method is that of Roy. In applying it, mixtures of 

 glycerin and water are made of different specific gravities, and 

 blood is dropped into these until one is found in which the drop 

 of blood will neither rise nor sink. Knowing the specific gravity 

 of this mixture, that of the blood, being the same, is also known. 



Color of Blood. Although blood is generally said to be red, 

 still this color is subject to considerable variation. Thus, venous 

 blood is variously described as bluish red, reddish black, deep 

 purple, dark purplish red, dark blue, and dark purple, while 

 arterial blood is a bright scarlet. The color of blood depends on 

 hemoglobin or its derivatives. In the blood of an animal that 

 has been suffocated, where the purplish or blackish color is most 

 pronounced, the coloring-matter is almost entirely hemoglobin, 

 while in arterial blood the oxyhemoglobin predominates, and in 

 ordinary venous blood there is a mixture of hemoglobin and oxy- 

 hemoglobin. 



When the coloring-matter passes out from the corpuscles into 



