THE RETICULAR FORMATION 



1283 



reticular formation also arise in the parietal (182), 

 lateral temporal (183), oriiital (275) and paraoccip- 

 ital (58, 180) areas, but these regions were found to 

 be relatively poor sources of such fibers (230). Re- 

 cently, Adey et al. (2) have demonstrated important 

 reticular connections from rhinencephalic structures, 

 principally the hippocampus and entorhinal region. 



Some of these corticoreticular fibers travel in the 

 corticospinal and corticobulbar tracts en route to the 

 reticular formation (222). Ramon y Cajal (222) 

 found them leaving the pyramid in particularly large 

 numbers above the olives at the level of the facial 

 nucleus. Rossi & Brodal (230) describe similar dense 

 collections of corticifugal fibers ending in two well- 

 circumscribed areas; one of these zones was located in 

 the lateral pontine tegmentum and the other resided 

 in the medulla near Olszewski's nucleus reticularis 

 gigantocellularis. Other routes exist through which 

 cortical neurons reach the reticular formation, but 

 probably because these fiber connections are diffuse 

 anatomical evidence concerning them is meager. 



An important anatomical and functional relation- 

 ship is known to e.xist between the reticular formation 

 proper and the mid-line and intralaminar nuclei of 

 the thalamus, these latter structures being considered 

 the cephalic portion of the reticular system. Most of 

 the data indicating that these thalamic structures re- 

 ceive important direct contact from afferent or corti- 

 cofugal neurons, however, stems from physiologic 

 rather than anatomic study; at least fiber degenera- 

 tions have not been described in these nuclei after 

 cortical excisions as they have in the reticular forma- 

 tion itself unless portions of the rhinencephalon were 

 included in the decortication (92, 201). Rhinen- 

 cephalic links with the thalamus and reticular forma- 

 tion have been followed from the hippocampus to the 

 intralaminar thalamic nuclei (201) and through the 

 fornix into the midbrain tegmentum (92). Adey et al. 

 (2) found connections between the entorhinal cortex 

 and reticular formation, and Nauta (201) described 

 reticulopetal axons from the septal region. 



Anatomical connections between the basal ganglia 

 and the central brain stem have not been reported, 

 although a major portion of pallidal outflow is known 

 to enter the ventralis lateralis (208), the ventralis 

 anterior or both (223). However, important reticulo- 

 petal fiber tracts are known to emanate from the 

 cerebellum. In 1924, Allen (7) reported that neurons 

 in the fastigial nuclei sent axons to the reticular forma- 

 tion and Rasmussen (224) later confirmed this find- 

 ing. Recently Sprague et al. (259J and others (179, 



268) have described important bilateral connections 

 between vermal fastigial structures and the reticular 

 formation and intralaminar thalamic nuclei. 



Central Brain Stern 



Ramon y Cajal was struck by the marked varia- 

 bility in size of neurons in the reticular formation, 

 some being only 12 to 14 /u in the longest diameter, 

 while others were as large as 90 11. Presumably, then, 

 if soina size relates to axon length, the reticular 

 formation is constructed in a manner which permits 

 conduction both in relatively short steps and in long 

 strides. The soma size can be related further to axon 

 diameter and conduction velocity; hence, both slowly 

 and rapidly conducting elements are present. Also, 

 Ramon y Cajal noted that axons often crcssed the 

 mid-line and ramified in all directions both ipsi- and 

 contralaterally. Recently, the Scheibels (239, 240) 

 have confirmed and extended these observations of 

 Ramon y Cajal by describing axons which divide, 

 sending one branch cephalad and one caudad. Each 

 branch subsequently- delivers itself of a myriad of 

 collaterals which appear capable of extending long 

 distances. This organizational pattern suggests 

 strongly that a single reticular cell may be capable of 

 exerting its influence both upwards towards the brain 

 and downwards into the spinal cord. It suggests fur- 

 ther that these influences may be exerted bilaterally, 

 that they may be conducted rapidly or slowly and 

 that they may be extended widely through the large 

 number of collaterals they exhibit. 



Anatomical connections from the reticular forma- 

 tion to the centrum medianum nucleus of the thala- 

 mus and to the subthalamus have been described by 

 Lewandowsky (153). More recently, the observations 

 of Russell & Johnson (234), of Papez (209) and of 

 Whitlock & .Schreiner (285) indicate that cephalic 

 conduction occurs in relatively well-marked fiber 

 bundles called the lateral reticulothalamic, tegmental 

 and tectothalamic tracts to the mid-line and intra- 

 laminar nuclei of the thalamus (285). Brodal & Rossi 

 (42) found that cells projecting cephalically were scat- 

 tered throughout the reticular formation but were 

 abundant in its medial two-thirds, particularly at the 

 level of the rostral third of the inferior olive and at the 

 level of the abducens nerve. Doubtless, important 

 cephalic conduction is mediated by multisynaptic 

 neuron systems scattered throughout the central brain 

 stem from this diffusion of reticular cells. 



