[Illustration: Fig. 2.--_A portion of the spinal cord with its nerves_ (after Bernard). The left-hand figure shows the anterior side; the right-hand the posterior. A the anterior, and P, the posterior root, they meet at _g_, the ganglion; _c_ and _d_ are filaments connecting two posterior roots.] 5. There are thirty-one pairs (sometimes thirty-two) of such nerves--namely, eight cervical, twelve thoracic, five lumbar, five sacral, and one (or two) coccygeal. Figs. 3 to 6 represent transverse sections, which display the entrance of the roots of the nerves into the anterior and posterior horns. 6. Similar masses of gray substance in the _Medulla Oblongata_ (which is the name given to the cord when it passes into the skull)[83] are supposed to be the origins of some other nerves (the cranial). [Illustration: Fig. 3.--_Transverse section of one half of the spinal cord in the lumbar region_ (after Kölliker). _a_, anterior root entering the anterior gray horns, _m_ and _l_, where cells are clustered; _c_, central canal; _d_ and _e_, the anterior and posterior commissures uniting the two halves of the cord; _b_, posterior root entering the posterior gray horn.] [Illustration: Fig. 4.--_Transverse section of both halves of the cord, cervical region._ _a_, Fissure separating the anterior columns; _b_, fissure of the posterior.] [Illustration: Fig. 5.--_Transverse section of the cord in the dorsal region._] [Illustration: Fig. 6.--_Transverse section in the lumbar region._] Although the Medulla Spinalis is unquestionably continued as the Medulla Oblongata, the arrangement of its tissues here becomes gradually changed, and so complicated that it baffles the scalpel. Anatomists are, however, agreed on the one point of fundamental importance to us here--namely, that there is only a rearrangement, not a new tissue. Accepting the artificial division into two organs, we may say that their functions are different, inasmuch as they are different in their anatomical connections--they innervate different parts; but as nerve-centres they have one and the same property. On its posterior surface the Medulla Oblongata opens as the _fourth ventricle_. It is then no longer a closed canal, but an expansion of the spinal canal, which is covered by the Cerebellum. On its anterior surface projects the _pons varolii_. Figs. 7 and 8 represent these. [Illustration: Fig. 7.--_Back, or upper view of the Medulla Oblongata as it continues the Med. Spinalis._ 1, Section of the thalami; 2, corpora quadrigemina (the two lower bodies are imperfectly represented in the engraving); 3, section of the crura cerebelli; 4, the fourth ventricle; 5, the restiform bodies; 6, the calamus scriptorius. ] [Illustration: Fig. 8.--_Front, or under view of the Med. Oblong._ 1, Optic nerves cut off at the chiasma; 2, crura cerebri; 3, pons varolii; 4, olivary bodies; 5, anterior pyramids; 6, spinal columns. ]