240 INTRODUCTION TO NEUROLOGY 



conception of Flourens that there is a minute "vital node" under the lowest 

 point of the fourth ventricle which is the respiratory center must be aban- 

 doned. Later the fasciculus solitarius was identified as the "respiratory 

 tract," and the nucleus associated with this tract was regarded as the 

 respiratory center, but further experiment has shown that this is not an 

 exact statement of the case. Some physiological experiments have sug- 

 gested that the respiratory rhythm is maintained by a center in the 

 reticular formation of the vagus region ventrally of the fasciculus solitarius. 



It has recently been shown, as stated above, that afferent visceral fibers 

 from the lungs whose cell bodies lie in the vagus ganglion enter the fascicu- 

 lus solitarius, and it is known that from the nucleus of this tract a "tractus 

 solitario-spinalis " (Fig. 112) descends into the motor centers of the upper 

 segments of the spinal cord. This descending visceral spinal tract probably 

 plays some part in the regulation of respiration, though not the chief role. 

 Rain6n y Cajal and Kappers believe that, while the upper part of the nucleus 

 of the fasciculus solitarius has nothing to do with respiration, the lower end 

 of this nucleus (commissural nucleus of Cajal, see Figs. 71, 112, and 114) is a 

 true respiratory center. Ram6n y Cajal, in fact, thinks that this nucleus 

 serves both for reflexes excited by the sensory pulmonary nerves and also for 

 the normal respiratory rhythm excited by carbon dioxid in the blood. 

 This hypothesis is not supported by direct physiological experiment, and for 

 the present we must content ourselves with the statement that the true 

 respiratory center has not been accurately located anatomically. Figure 

 112 may be regarded as a true picture of the essential relations of the 

 respiratory nerves, with the reservation that the position of the respiratory 

 center is not precisely known. 



There is also a reflex center for the regulation of respiration in the medial 

 wall of the thalamus and others have been described in different parts of the 

 brain stem. The entire respiratory mechanism is also under partial volun- 

 tary control from the cerebral cortex. 



While many features of the central respiratory mechanism 

 remain obscure, it seems evident that the location of the chief 

 respiratory center in the "visceral area" of the lower part of the 

 medulla oblongata instead of the portions of the spinal cord 

 directly connected with the respiratory muscles is a survival of 

 the ancestral condition found in fishes, where the entire respira- 

 tory function is carried on by a visceral apparatus (gills) inner- 

 vated from the vagus region. 



Organs of Digestion. Hunger seems to be a complex in which 

 at least three factors are present: (1) Specific hunger pangs due 

 to waves of muscular contraction in the stomach (Cannon, 

 Carlson) ; (2) appetite, or craving for food regardless of the state 

 of the stomach; (3) general malaise from starvation of the tissues 

 and weakness. Appetite may persist after section of the vagus 

 nerves and is probably a sensation distinct from the hunger 

 pangs. 



