656 THE NERVOUS SYSTEM. 
mater of the posterior fossa of the base of the skull. It probably derives its fibres 
from the communication with the first and second cervical nerves. 
The descending hypoglossal nerve (n. descendens) is the chief branch given 
off in the neck. It arises from the hypoglossal nerve as it crosses the internal 
carotid artery, and descends in the anterior triangle in front of the carotid sheath. 
It is joined about the middle of the neck by the descending cervical nerve (from the 
second and third cervical nerves). By their union the hypoglossal loop (ansa hypo- 
glossi) is formed, from which branches are distributed to the majority of the mfra- 
hyoid muscles—both bellies of the omo-hyoid, the sterno-hyoid, and the sterno-thy- 
roid. The descending hypoglossal nerve derives its fibres from the communication 
to the hypoglossal nerve from the loop between the first and second cervical nerves ; 
so that the ansa hypoglossi is made up of fibres of the first three cervical nerves. 
The nerve to the thyro-hyoid muscle is a small branch which arises from the 
hypoglossal nerve before it passes beneath the mylo-hyoid muscle. It descends 
behind the great cornu of the hyoid bone to reach the muscle. When traced 
backwards, this nerve is found associated with the loop between the first and 
second cervical nerves. 
The lingual branches of the hypoglossal nerve are distributed to the hyo-glossus, 
genio-hyoid, and genio-hyo-glossus, and to all the intrinsic muscles of the tongue. 
The nerve to the genio-hyoid is said to be derived from the loop between the 
first and second cervical nerves. It is not known if these two nerves are im- 
plicated in the innervation of the proper muscles of the tongue, but it appears 
certain that the muscles named—the genio-hyoid, thyro-hyoid, sterno-hyoid, omo- 
hyoid, and sterno-thyroid—are not supphed by the hypoglossal, but only by cervical 
nerves, the genio-hyoid by the first two, the other muscles by the first three cer- 
vical nerves. 
THE DEVELOPMENT OF THE CRANIAL NERVES. 
Omitting the first and second nerves, there is an obvious likeness in the de- 
velopment of the cranial nerves to the formation of the dorsal, afferent or sensory, 
and the ventral, efferent or motor, roots of the spinal nerves. The afferent roots 
arise from collections of cells which bud off from the alar lamina of the brain, homo- 
logous with the dorso-lateral part of the spinal cord. These cells give rise to 
central and peripheral processes, like the similar processes from the dorsal ganglia 
of the spinal nerves, producing on the one hand the root fibres connected with the 
brain, and on the other hand the fibres of the nerve proceeding to the periphery. 
The efferent roots, like the ventral roots of the spinal nerves, arise as the peripheral 
processes of neuroblasts located in the basal lamina of the primitive brain, which is 
homologous with the ventro-lateral portion of the spinal cord. The efferent nerves 
may be separated into two series, according as they arise from the mesial or lateral 
parts of the basal lamina. The third, fourth, sixth, and twelfth nerves arise from 
the mesial part of the lamina; the efferent fibres of the fifth, seventh, ninth, tenth, 
and eleventh nerves arise from the lateral part. 
The olfactory nerve is associated in its development with the formation of the nasal 
pit and the olfactory bulb. 
The olfactory nerves are developed from the epithelium of the nasal pit. These 
cells furnish neuroblasts—cells with peripheral and central processes, which form a collec- 
tion of cells—the olfactory ganglion. From this ganglion the processes of the neuroblasts 
extend peripherally and centrally. The peripheral processes produce the so-called olfac- 
tory nerves, which become connected with the olfactory epithelium. The central pro- 
cesses extend from the olfactory ganglion to the brain, applying themselves to the olfac- 
tory bulb, to which they become connected in the second month. The olfactory ganglion 
becomes incorporated at the same time with the olfactory bulb. 
The optic nerve is developed wholly from the brain. Its formation begins with 
the outgrowth of the optic vesicle, a paired hollow outgrowth from the ventral surface of 
the thalamencephalon. The epiblastic invagination of the lens, growing inwards from 
the surface, causes the collapse of the vesicle and its conversion into the optic cup, the 
narrow tube connecting the vesicle to the brain becoming the optic stalk. This stalk 
