THE CRANIAL NERVES. 249 



each crescent advance towards the shallow lateral longitudinal lines 

 seen on the surface of the cord. Again, quite at the lower end of the 

 cord, the gray matter loses its crescentic arrangement, and forms a 

 slight rounded mass ; and the white matter, which progressively de- 

 creases in quantity from above downwards, at length disappears, and 

 at the extreme point, only gray matter exists. The white matter 

 altogether, forms about seven-eighths, and the gray one-eighth, of the 

 entire substance of the cord. The anterior horn of each crescent is 

 short and thick ; whilst the posterior horn is long and narrow. The 

 posterior horn at its hinder and outer part, is more transparent than 

 elsewhere, forming the so-called gelatinous portion, in which the nerve 

 cells are large and multipolar ; near the root of this cornu, on its inner 

 side, is another peculiar column of gray matter, named the vesicular 

 column. Throughout the whole extent of the cord, the gray matter 

 of the one side is connected with the gray matter of the other, by 

 what is called the central gray commissure. In the centre of this 

 commissure, is the central canal of the spinal cord, a passage measur- 

 ing about T Jo tn f an i ncn 1U diameter, and lined with ciliated col- 

 umnar epithelium. In front of, and behind, the gray commissure, is a 

 thin layer of white substance, which forms the bottom of the anterior 

 and posterior median fissures. The cervical and lumbar enlargements 

 of the cord, are principally caused by an increase of the gray matter. 



Taking a general view of the brain and spinal cord, we see that the 

 cerebrum and cerebellum are composed of gray matter externally, 

 which gray matter, especially in the case of the cerebrum, forms a 

 large proportionate share of its mass. On the other hand, the cerebral 

 and cerebellar peduncles, the pons Varolii, the medulla oblongata, and 

 the spinal cord, are composed of white matter externally, and of gray 

 within. In the interior of the cerebrum and cerebellum, there is 

 chiefly white substance ; whilst in the interior of the cerebral and 

 cerebellar peduncles, pons, medulla, and cord, we meet with gray 

 matter. Special gray masses, however, are also found in the lower 

 part of the interior of the cerebral hemispheres, resting upon the 

 diverging and expanding peduncles, viz., the corpora striata, optic 

 thalami, corpora quadrigemina, and corpora geniculata. The cere- 

 bellum also has its special internal gray corpora dentata, and so have 

 the olivary bodies of the medulla oblongata. The course of the white 

 fibres in the medulla oblongata and the spinal cord, and their connec- 

 tions with the masses of gray matter within those parts, and with the 

 roots of the nerves, will be hereafter described. 



The cerebro-spinal nerves. These nerves are divided into two sets, 

 called the cranial nerves, and the spinal nerves, according as they 

 pass to their respective destinations through openings in the base of 

 the cranium, .or through the intervertebral foramina between the 

 several vertebrae. 



The cranial nerves. These consist of nine pairs. The parts usually 

 defined as the first pair, or the olfactory nerves, Fig. 60, 1, are really 

 extensions of the cerebrum, and should be named the olfactory lobes. 

 They are attached to the anterior cerebral lobe by three roots. The 

 true olfactory nerves, or nerves of smell, arise from these lobes ; they 



