624 THE NERVOUS SYSTEM 



nervous tissue is indicated even by the relatively gross means, figure 357, 

 shown in the histological preparations of nerve cells. 



Somnambulism and Dreams. What we term sleep occurs often in very 

 different degrees in different parts of the nervous system; and in reference to 

 some parts the expression cannot be used in the ordinary sense. For example, 

 during the most profound cerebral sleep the medulla is discharging rhythmic 

 nerve impulses to maintain respiratory movements. 



The phenomena of dreams and somnambulism are examples of differing 

 degrees of sleep in different parts of the cerebro-spinal nervous system. In 

 the former case the cerebrum is still partially active; but the activity is no 

 longer corrected by the reception, on the part of the sleeping sensorium, of 

 impressions of objects belonging to the outer world. Neither can the cerebrum, 

 in this half-awake condition, act on the centers of reflex action of the volun- 

 tary muscles, so as to cause the latter to contract a fact within the painful 

 experience of all who have suffered from nightmare. 



In somnambulism the higher centers are capable of co-ordinating that train 

 of reflex nervous action which is necessary for progression, while the nerve 

 center of the muscular equilibrium sense (in the cerebellum?) is, presumably, 

 fully awake; but the sensorium is still asleep, and impressions made on it are 

 not sufficiently felt to rouse the cerebrum to a comparison of the difference be- 

 tween mere ideas or memories and sensations derived from external objects. 



VII. THE SYMPATHETIC SYSTEM. 



The fact has already been emphasized that the sympathetic system of 

 nerves is an organic and constituent part of the nervous system. This fact 

 should be constantly kept in view in the discussions that follow. 



Organization and Distribution. The sympathetic system consists 

 of those collections of nerve cells or ganglia lying outside of the brain and 

 cord (exclusive of the root ganglia), and the nerve tracts connecting them 

 with one another and with the cerebro-spinal axis. Its parts that should be 

 mentioned are: i. a double chain of ganglia and fibers, which extends 

 from the cranium to the pelvis, along each side of and in front of the vertebral 

 column, and from which branches are distributed both to the cerebro-spinal 

 system and to other parts of the sympathetic system. With these may be 

 included the small ganglia in connection with those branches of the fifth 

 cerebral nerve which are distributed in the neighborhood of the organs of 

 special sense, namely, the ophthalmic, otic, spheno-palatine, and submaxillary 

 ganglia. 2. Various ganglia and plexuses of nerve fibers which give off 

 direct branches to the thoracic and abdominal viscera, the chief of such 

 plexuses being the cardiac, solar, and hypogastric plexuses. In intimate 

 connection with these are many secondary plexuses, as the aortic, spermatic, 

 and renal. Fibers pass from the prevertebral chain of ganglia and from 

 the cerebro-spinal nerves to these plexuses. 3. Various ganglia and plexuses 

 in the substance of many of the viscera, as in the stomach, intestines, and 



