PHYSIOLOGY OF ORGANS _ 163 



cerebrum. This statement is not entirely true since certain muscles are 

 connected to neurons in both sides of the brain. 



The cells of the cortex are mostly of the type illustrated in Fig. 121. 



Functions of the Cerebellum. — Although great progress has been 

 made in the Study of the cerebellum there still exists much difference of 

 opinion regarding its function. In some manner it regulates the mechan- 

 ism of automatic movements. It has the effect of coordinating volun- 

 tary movements, especially the complex movements of equilibrium and 

 locomotion. It is not believed that the cerebellum is responsible for 

 a high degree of consciousness, if any. Unlike the relations of the 

 cerebrum to muscles, each half of the cerebellum is connected with the 

 organs on the same side of the body with itself. The cells of the cere- 

 bellum are of several types and the arrangement of fibers is very complex. 



Functions of the Medulla Oblongata. — This portion of the brain 

 includes centers which control the activity of the organs of circulation 

 and respiration. Through the vagus (tenth cranial) nerve it has an in- 

 hibitory effect on the heart. It also controls the movements and 

 secretions of the alimentary canal. 



Important Nerves Not Discussed. — To name the twelve pairs of 

 cranial nerves and state their distribution to the organs of the head and 

 trunk would lead beyond the limits of this book. For these and many 

 other facts in regard to the nervous system reference should be made to 

 textbooks on anatomy, physiology, and psychology. 



Functions of the Sympathetic or Autonomic Nervous System. — Neu- 

 rons from this system supply the smooth muscle of the viscera, the car- 

 diac muscles, and the erector muscles of the hair. Blood vessels and 

 glands are also supplied by nerves of this system. Impulses carried by 

 its cells originate in the central nervous system in centers lower than 

 the cerebral cortex. These impulses are involuntary. 



Sensory Nerve Endings.— The dendrites of the afferent neurons 

 originate in the skin and in the tissues of the body, especially in epithe- 

 lium, muscle and connective tissues, and in organs of special sense. 

 Each neuron when stimulated at its peripheral ending conveys an impulse 

 to the central nervous system where it usually gives rise to an impulse that 

 finds expression in the contraction of muscle or discharge of a glandular 

 secretion. Besides initiating this reflex action the stimidus may cause an 

 impulse to be carried over an appropriate path to a higher nervous center 

 where it enters the consciousness of the subject producing a sensation of 

 some sort. The particular kind of sensation aroused by the stimulation 

 depends not so much upon the character of the stimulus as it does upon 

 the kind of sense organ involved and the nature of the central connections. 

 The classes of sensations which originate from stimulation of body sur- 

 faces and tissues other than the organs of special sense are those of pres- 

 sure, warmth, cold, pain, strain sensation from muscle and tendon, and 

 various organic sensations. For the mediation of each of these sensations' 



