CENTRAL AUTONOMIC MECHANISMS 



957 



"By a high spinal transection the splendid motor ma- 

 chinery of the vertebrate is practically as a whole and 

 at one stroke severed from all the universe except its 

 own microcosm and an environmental film some 

 millimeters thick immediately next its body. The 

 deeper depression of reaction into which the higher 

 animal as contrasted with the lower sinks when made 

 spinal signifies that in the higher types more than in 

 the lower the great distance-receptors actuate the 

 motor organ and iinpel the actions of the individual. 

 The deeper depression shows that as the individual 

 ascends the .scale of being the more reactive does it 

 become as an individual to the circumnambient uni- 

 verse outside itself. It is significant that spinal shock 

 hardly at all affects the nervous reactions of the intero- 

 ceptors (visceral system); and that it does not affect 

 the intero-ceptive arcs appreciably more in the 

 monkey than in the frog. . . . Not that in the highest 

 animal forms the 'distance-receptor' merely l)er se 

 has necessarily reached more perfection or more 

 competence than in the lower. ... It is that in the 

 higher types there is based upon the 'distance-recep- 

 tors' a relatively enormous neural superstructure 

 possessing million-sided connections with multitudi- 

 nous other arcs and representing untold potentialities 

 for redistribution of .so-to-say stored stimuli by associa- 

 tive recall. The development and elaboration of this 

 internal nervous mechanism attached to the organs of 

 distance-reception has, so far as we can judge, far out- 

 stripped progressive elaboration of the peripheral re- 

 ceptive organs themselves. Adaptation and improve- 

 ment would seem to have been more precious assets 

 in the former than in the latter" (154). 



Of these adaptive phenomena at both spinal and 

 cerebral levels we are, perhaps, acquiring some glim- 

 merings. Of the 'enormous neural superstructure' we 

 have more records of detailed observations on neo- 

 pallial, rhinencephalic, diencephalic and bulbar mech- 

 anisms than were available to Sherrington, but one 

 wonders if we have deeper insight. 



cord. Collaterals from these pathways connect with 

 organized nuclear groups of neurons, cell stations for 

 cranial nerves, bundles of fibers concerned with modu- 

 lation of these \ arious functions, and many other in- 

 tegrated systems. These known masses of fibers and 

 neurons are embedded in a matri.x of incredible com- 

 plexity, which in recent years has attracted increasing 

 attention as the 'reticular formation of the brain 

 stem.' Segundo (150) has recently summarized briefly 

 the knowledge of the structure of the reticular forma- 

 tion and cites Ramon y Cajal's conclusions as to the 

 constitution of the dense interstitial plexus which 

 forms this reticulum. Thus we have fibers coming 

 from the spinal cord, from the inedial lemniscus which 

 is constituted of sensor) fibers of the second order 

 and which contributes collaterals to reticular cells; 

 motor pathways, including collaterals from the pyram- 

 idal tract; fibers from interstitial motor cells; and 

 fibers from the cerebellum, acoustic nuclei, and the 

 colliculi of the midbrain. A careful investigation of 

 the structure of this area, which extends from the 

 lower part of the bulb into the diencephalon, has been 

 initiated by Schcibel & Scheibel (148) who have 

 found collaterals from the main ascending and 

 descending tracts passing into the reticular formation 

 and setting up complex axodendritic-somatic synapses 

 which seem to make possible a potential interaction 

 between dendritic and .somatic neuronal fields. The 

 axons of the reticular formation have very large po- 

 tential areas of interaction with other neurons, 

 theoretically with as many as 27,000 others, although 

 of course the number undoubtedly is much less in 

 many cases. The axons of the reticular formation 

 neurons and those axons passing through the reticular 

 formation have widespread connections, with many 

 collaterals and bifurcations (fig. 2). The reticular 

 formation has connections with the hypothalamus, the 

 subthalamus and thalamus, and possibly even with 

 the cortex. Besides the ascending systems there is 

 evidence that the descending systems are just as 

 complex. 



AUTONOMIC MECHANISMS OF SUBDIENCEPHALIC 

 BRAIN STEM 



Reticular Fnrmaliori 



It is axiomatic that progressive encephalization in 

 the nervous system has produced increasing ana- 

 tomical complexity as one proceeds from one level to 

 another. Thus through the brain stem we have exten- 

 sive ascending and descending pathways for interre- 

 lating the functions of the higher regions and the spinal 



Medulla Oblongata and Pons 



In this maze are embedded the groups of neurons 

 which are known to be related to efferent outflow 

 through the cranial nerves and through the spinal 

 cord. It seems unnecessary to review the structure 

 and location of these entities as they are well described 

 in various textbooks of neuroanatomy and in such 

 monographs as that of Mitchell (122). While these 

 nuclear groups have definite and known functions 



