458 THE ORGANS 



(Bruce). This group probably innervates the striated voluntary (somatic) 

 muscles of the trunk. The mesial group is in part of the cord subdivided into 

 a ventro-mesial and dorso-mesial group, the latter being present in the first, 

 sixth and seventh cervical, thoracic (except first), and first lumbar segments, 

 (b) Lateral group, present in the cervical, first thoracic, and lumbo-sacral re- 

 gions. This group innervates the muscles of the extremities and exhibits the 

 following subdivisions: An antcro-lateral (C4 to C8, L2 to S2), a postero-lateral 

 (C4 to C8, L2 to S3) and a post-postero-lateral (C8 to Thi, Si to S3). There is 

 also a central group (L2 to S2) and a small anterior group (Li to L4). The exact 

 muscle groups innervated by these cell groups, respectively, have not yet been 

 definitely determined. Other special cell groups are Xhc phrenic group (C4), cen- 

 trally located, cilio-spinal and other cells (C8 to Th2, to sympathetic ganglia 

 which send fibres to dilator pupillae and blood-vessels of head), and the spinal 

 accessory (Ci to C6). The latter is located laterally and innervates the sterno- 

 mastoid and trapezius muscles. In the lumbo-sacral cord below the fourth 

 lumbar there is also a medio-ventral splanchnic group which together with the 

 lateral horn group of the sacral cord furnishes the preganglionic fibres emerging 

 from the sacral cord. 



For the determination of the destination of the axones of the efferent root 

 cells the method of studying the changes in the cell body (Nissl stain) in definite 

 lesions of the peripheral fibres (axonal degeneration, see Chapter X) is used. 



Arrangement of Fibres (Fig. 320). — With the low- and high-power objectives 

 the course of the transverse (i.e., longitudinally cut) nerve fibres should be care- 

 fully studied in Weigert and Cajal preparations. These fibres pass from gray 

 to white matter or vice versa, and are in general (a) root fibres entering or leaving 

 the cord; (b) either axones of column cells in the gray passing out into the white 

 there to become longitudinal fibers by turning or splitting, or they are the col- 

 laterals and terminals of the fibres of the white matter entering the gray to 

 terminate there. 



The arrangement of these fibres should be carefully studied in all parts of the 

 section (Weigert and Cajal), taking one field at a time. In the dorsal part of the 

 cord, the dorsal roots can be seen entering. From their lateral portion fine 

 fibres detach themselves and enter the zone of Lissauer, the fibres of which are 

 largely composed of their short ascending and descending arms. Most of the 

 fibres of the root pass along the dorsal and mesial side of the dorsal horn, 

 forming the zone of entry of the dorsal roots. By bifurcating (not visible in the 

 preparation) they become the majority of the longitudinal fibres of the dorsal 

 funiculus. From the entering root fibres and fibres of the dorsal funiculus, 

 bundles of fine fibres (collaterals and terminals) pass radially through the 

 gelatinous substance of Rolando or sweep around its mesial side and enter the 

 gray. Some of these terminate in the gelatinous substance of Rolando (Golgi 

 preparations), some form part of the dense plexus of fibres in the caput and ter- 

 minate there, others can be traced to the intermediate gray and, in some cases, 

 some ("direct reflex collaterals") can be traced to the ventral horn. It will be 

 noted that not many come from the mesial part of the dorsal funiculus. Col- 

 laterals from the zone of Lissauer enter the gelatinous substance of Rolando. 

 In the middle part of the cord there is a similar interchange of fibres between 



