130 ORIGIN AND DISTRIBUTION OF THE NERVES. 



and gelatinous matter. Its dura mater is derived from that 

 which covers the brain : in being continued through the fo- 

 ramen magnum, the membrane is contracted into a cylindrical 

 sheath, which loosely encases the marrow, and is generally 

 described under the denomination of theca vertebralis. The 

 arachnoid membrane and pia mater have the same relation to 

 the marrow that the same membranes have to the brain, of 

 which they may be considered prolongations. 



ORIGIN AND DISTRIBUTION OF THE NERVES. 



The nerves being symmetrical in number and distribution 

 on either side of the body, take their origin in pairs, and 

 these pairs are numbered, and so distinguished from one 

 another, according to the order in which they arise. There 

 are forty-six pairs of nerves ; ten, coming from the brain, are 

 distinguished as the cerebral nerves ; thirty-six, from the 

 spinal marrow, denominated the spinal nerves. 



CEREBRAL NERVES. 



First pair, or olfactory nerves, arise from the corpora straita, 

 along the posterior borders of which bodies the medullary 

 bands or roots of them may be traced as high up as the mid- 

 dle lobes of the cerebrum. These are the largest of the cere- 

 bral nerves, are bulbous at their origin, pulpy in texture, and 

 exhibit, when cut into, comparatively to their size, large 

 cavities, which are walled in by a layer of medullary matter, 

 enclosed within a thinner one of cortical substance. 



Second pair, or optic nerves, arise from the thalamia nervo- 

 rum. They leave the cranium through the optic foramen, 

 and pass to enter the globe of the eye, within the interior of 

 which it expands, and forms the retina. In its whole course, 

 it is enclosed within a sheath prolonged from the dura mater. 



Third pair, or motores oculorum, take their origin by sev- 

 eral filaments, from the inward parts of the crura cerebri. 

 The trunk of the nerve first runs obliquely outward, across 

 the back of the crus, then turns downward and enters the 



