The sensory roots of the spinal nerves terminate in the posterior vesicular columns of the spinal cord, and the sen sory cranial nerves in masses of vesicular matter in the outer part of the floor of the fourth ventricle. The motor roots of the spinal nerves are connected with the anterior vesiclar columns of the cord, and the motor cranial nerves with the motor centers along the middle of the floor of the ventricle.
The roots of the spinal nerves emerge from the sides of the spinal cord; the anterior roots in a line about one-third of the distance laterally on each side of the anterior median fissure, and the posterior roots also about one-third of the distance outward upon each side of the posterior median fis sure, dividing the spinal cord on each side into three columns, anterior, lateral and posterior. Each column of the cord is subdivided into tracts which can be traced through the medulla to join with the several divisions of the pens Varolii described, and with the restiform bodies or inferior peduncles of the cerebellum.
The spinal cord is variable in size iu different localities, due to the difference in the volume of the. spinal nerves received by it at certain points. The lumbar enlargement is near the lower end of the cord and receives the nerves from the lower extremities. The cervical enlargement extends from the third cervical to the first dorsal vertebra, receives the nerves from the upper extremity and is somewhat larger than that of the lumbar region. The medulla oblongata is the uppermost enlargement of the spinal cord, it lies within the cranial cavity and is sometimes called the bulb. This part of the spinal cord contains the centers which control the func tions of respiration and circulation, and is therefore one limb of the tripod which immediately sustains life. It also co ordinates the muscles that perform the acts of deglutition and speech, besides other important functions. Symptoms indicating defection of this part of the central nervous sys tem are of grave import, and are spoken of as bulbar symp toms, bulbar paralysis, etc. Certain diseases originate from functional or organic disturbance of the medulla, therefore a familiarity with its anatomy is fundamental to the diagnosis of these diseases, and also assists in the early recognition of those symptoms which denote the approach of death. The rela
tions of its tracts to the several nerves of distinct function provide certain combinations of symptoms by which the anato mist can determine the exact situation and extent of a lesion.
In order that the relations and the continuity of the col umns and tracts of the spinal cord with those of the medulla oblongata, and the tracts of the latter with the divisions of the pons \Tarohi and with the cerebellum, may be more clearly understood, we will locate and name, first, the tracts of the spinal cord, then those of the medulla, and afterward associate them with each other, and with the divisions of the pons which have before been described.
The spinal cord proper, as stated, is that which extends below the decussation of the anterior pyramids of the me dulla oblongata. It is uniform in structure, size and appear ance except the enlargements of its cervical and lumbar regions, and that its fascicular tracts, or the white substance diminishes in quantity from above downward.
The, grey columns and the central canal, or ventricle of the cord, have been described; the former as being continu ous with the grey matter in the floor of the fourth ventricle, and the latter as a, part of the ventricular system which extends through the whole length of the cerebro spinal axis.
It remains to trace the tracts which form the white col umns of the cord and to describe their relations.
The relation of these tracts will be more easily- under stood by an examination of a transverse section of the spinal cord as represented in figure 38, which exhibits the division of the cord into three columns by tlie anterior and posterior roots of the spinal nerves.
Each of the, three columns of the cord are divided into tracts, the anterior and the posterior each, into two, and the lateral into four. Each of these tracts are endowed with a specific function, and are arranged in bundles which can be separated in part by careful dissection, traced by the degeneration observed as the result of disease, and also by the observation of the development of the embryo.