THE SPINAL NERVES. 717 



if this reflex impulse from the posterior roots for the increase of the irritability 

 were removed. Obviously, the irritation required for the excitation of an already 

 slightly irritated nerve-fiber need be less than for a similar fiber that is not irri- 

 tated, as, in the first instance, the existing irritation is added to that which is 

 in constant action. 



The anterior roots of the spinal nerve supply with centrifugal fibers : 



1 . All striated muscles of the trunk and of the extremities under the 

 control of the will. Every muscle receives its motor fibers from several 

 anterior roots, and not from a single root; while every root distributes 

 fibers to a related group of muscles. 



The experiments made by Ferrier and Yeo on the anterior roots in apes have 

 shown, accordingly, that irritation of each root (in the brachial and lumbosacral 

 plexuses) induces a synergistic coordinated movement. Division of one root 

 failed also to cause complete paralysis of the muscles taking part in the combined 

 movement, but these had suffered only loss in strength. These experiments con- 

 firm pathological observations made on man. The fibers for functionally related 

 groups of muscles, for example flexors and extensors, arise from special circum- 

 scribed regions of the spinal cord. Thus the cervical and lumbar swellings of the 

 cord represent centers for highly coordinated muscular movements. 



2. The anterior roots supply, also, motor fibers to a number of organs 

 provided with unstriated muscle-fibers, such as the urinary bladder, the 

 vasa deferent ia, the uterus, the skin. 



3. Motor fibers for the unstriated muscles of the vessels, the vaso- 

 motors. 



4. Inhibitory fibers for the contraction of the vascular muscles 

 (known only in part) : vasodilators. 



5. Secretory fibers for the sweat. 



6. Trophic fibers for the tissues. 



The posterior roots contain the sensory nerves for the skin and the in- 

 ternal tissues, with the exception of the anterior part of the head, the 

 face and the inner portions of the head. They contain also the tactile 

 nerves for the cutaneous surfaces indicated. Irritations, exciting re- 

 flex action, are also conveyed to the spinal cord through the posterior 

 roots. 



Every sensory root gives fibers to different peripheral nerves. Every 

 posterior root corresponds to a circumscribed area of the skin, although 

 adjacent cutaneous areas overlap in part, so that probably every portion 

 of skin is innervated from at least two roots. Thus, for example, the 

 nipple is supplied with sensory fibers from the fourth and from the third 

 and fifth sensory thoracic roots. The areas even extend somewhat be- 

 yond the middle line of the abdomen and the back and into one another. 

 The innervational areas of the sensory nerve descend lower than those of 

 the nerve-fibers arising from the corresponding anterior roots. 



Fig. 247 and Fig. 248 illustrate the areas of distribution of the sensory nerves 

 of the extremities, Fig. 244 those of the sensory spinal branches on the head. 

 In cases of neuralgia and anesthesia the nerves involved can be readily determined 

 by comparison with these illustrations. 



There receive sensory nerves as follows: Heart and lungs from the vagus and 

 the upper thoracic nerves; stomach, small intestine, liver, spleen and pancreas 

 from the vagus and the middle inferior thoracic and upper lumbar nerves; adrenals, 

 kidneys, testicles, ovaries, uterus from the middle and lower thoracic and upper 

 lumbar nerves; rectum, prostate, penis, uterus, vagina from the sacral nerves and 

 the hypogastric plexus (from the lower dorsal and upper lumbar cord) . 



In the hen it is a remarkable fact that a few motor fibers pass out through the 

 posterior roots; also in some fish; with extreme rarity also in the frog; further in 

 the dog and the cat vasodilators (for the hind leg) ; in the frog motor nerves for 

 the unstriated muscles of the digestive tract and the urinary bladder. 



