370 ANATOMICAL TECHNOLOGY. 



The latter is more directly connected with the sMn and with the 

 slceletal muscles which are commonly voluntary. 



The former is chiefly distributed to the viscera and to the vascu- 

 lar system, and is regarded by some as merely a dependency of 

 the other. 



§ 997. Central and Peripheral Portlonn. — Provisionally adopting the arrangement 

 above indicated as at least convenient, each of these two great divisions of the nervous 

 system consists of a central and a peripheral portion. 'Ihe central portion of the sym- 

 pathicisa double chain of gang ia placed along the ventral aspect of the columna ver- 

 tebralis, and thus within the ventral or visceral somatic cavity. Two pairs of these 

 ganglia are shown in Fig. 109, that of the left side displaced dorsad. The peripheral 

 part comprises the branches from these ganglia, with plexuses situated among or upon 

 the viscera and vessels, and numerous small ganglia in the walls of the intestine and else- 

 where. The details of the arrangement of this system will be given in connection with 

 Fig. 107 and 109, where also will be pointed out the existence of rami communicantes 

 between the sympathic and the cerebro-spinal systems. 



The peripheral part of the cerebro-spinal system consists of nerves which occasionally 

 form plexuses (Fig. 104, 105), and present ganglia at points to be presently indicated. 



The central part of the cerebro-spinal system is lodged within the canalis neuralis s. 

 cerebro-spinalis — the cavities of the cranium (§ 491) and vertebr.il column (§ 479). It is a 

 continuous mass of alba and cinerea. The cranial portion is the encephalon or Irain, 

 and the spinal portion is the myelon or spinal cord. The dorsal aspect of the brain and 

 of part of the myelon is shown in Fig. 104. 



§ 998. The Myelonal Alba and Cinerea. — The cinerea of the myelon presents the form 

 of a column deeply fluted on four sides, dorsal, ventral and lateral. On a transection, the 

 appearance is approximately that of a leiter H, the ends of the two uprights being curved 

 laterad Hence these areas are commonly known as the cornua or hwns, two dorsal and 

 two ventral. 



The interspaces are occupied by the alba. Since, however, the myelon is nearly divided 

 into lateral halves by the fissurea, dorsal and ventral, what would otherwise be single dor- 

 sal and ventral columns of alba are double ; hence on each side there is a lateral column 

 and a dorsal and ventral one. These features are indicated upon Fijr. 109 and 118. The 

 general arrangement of alba and cinerea in the brain will be described in connection 

 with the structure of that organ in Chap. X. 



§ 999. Motor and Sensory Nerves.— 'Bj means of experiments, it has been ascertained 

 that certain nerves mainly or exclusively transmit sensory impressions, made upon the 

 parts to which their peripheral ends are distributed, toward the myelencephalon, that is, 

 centrad or centripetally ; while others convey motor impulses to muscles in the opposite 

 direction, peripherad or centrifugally. 



§ 1000. Motor and Sensory Boots.— A nerve, especially a large trunk, usually contains 

 some of both sets of fibers ; but near the junction of the trunks with the myelon, each 

 trunk divides into two roots, which are attached to the myelon upon its dorso-lateral and 

 ventro-lateral aspects respectively. Anatomically, these roots are dorsal and ventral (an- 

 terior and posterior), but physiologically, they differ as do certain of the cranial nerves ; 

 the dorsal ones transmit impressions centrad, and the ventral ones transmit motor impulses 

 peripherad. Hence they are commonly designated as the motor and sensory roots of the 

 spinal nerves. 



