Biology of Man 489 



lymph ducts and their enlargements, the lymph nodes, (2) in tissue 

 spaces (tissue sinuses) or cavities in various tissues, (3) in the pleural 

 cavity (around the lungs), (4) in the pericardial cavity (around the 

 heart), (5) in the peritoneal cavity (abdominal cavity), (6) in the peri- 

 neural cavities (spaces between the various linings of the brain and 

 spinal cord), and (7) in the lacteals or lymphatics which originate in 

 the small fingerlike villi of the intestine. Fats are absorbed from the 

 intestine by the lacteals and eventually placed in the blood stream. 



Human Blood Groups 



The various types of human blood are classified as (1) Groups A, B, 

 AB, and O, (2) Groups M and N, and (3) Rh positive and Rh negative. 

 These are considered later in this chapter under Inheritance of Human 

 Traits. 



VI. RESPIRATION IN MAN 



Respiration may be defined as the supplying of oxygen to all cells of 

 the body and the removal of carbon dioxide from them. Breathmg may 

 be defined as the rhythmic inhalation of air into the lungs and the 

 exhalation of carbon dioxide and other gases from them (Fig. 244). 

 The composition of inhaled (inspired) air and of exhaled (expired) 

 air is: 



inhaled air exhaled air 



(per cent) (per cent) 



Oxygen 20.96 15.8 



Carbon Dioxide 0.04 4.0 



Nitrogen 79.00 80.2 



Respiration is controlled by the respiratory center of that portion of 

 the brain called the medulla oblongata (Fig. 247), whose activity is in- 

 fluenced by nerve impulses over afferent nerves leading to it and by 

 chemicals which influence the center either directly or reflexly. During 

 inhalation the size of the thorax (Fig. 243) is increased by contraction 

 of the respiratory muscles, thereby decreasing the pressure within the 

 lungs and allowing the greater pressure of the external air to force it 

 into the lungs until the pressures are equalized. During exhalation the 

 size of the thorax is decreased by relaxation of the respiratory muscles, 

 thus forcing out a certain quantity of the air from the lungs and allowing 

 a certain amount to remain. In adults, during rest, the normal rate of 

 respiration varies from 12 to 20 per minute, although these figures may 

 vary with different individuals. A certain amount of respiration takes 



