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cerebro-spinal nervous system, it is certain that the cells and fibres of the sympathetic system possess a vital activity apart from their connexion with the central nervous system. In the us development of the sympathetic it is at least highly probable that a mesoblastic rudiment or Fr precursor forms the basis of the sympathetic system, which is secondarily joined by nervefibres from the roots of the spinal nerves.
Morphologically this part of the nervous system is essentially a longitudinal cord or column, associated with involuntary muscles and glandular tissues, and particularly related to the organs in the splanchnic area. Like other longitudinal structures in the body, and especially like the In organs of the splanchnic area, the sympathetic system is not truly segmental. The sympathetic trunk is only quasi-segmental, the segmentation being attributable to its junction with the visceral branches of the spinal nerves. The peripheral branches from the sympathetic trunk are by no means segmental; even the gray rami are not properly metameric, but, like the ganglia, assume a segmental character in consequence of their connexions with the spinal nerves.
The phylogenetic relation of the sympathetic and the cerebro-spinal elements in the system it is impossible to determine. It may be that the sympathetic system is the representative of an ancient architecture independent of the cerebro-spinal nervous system, the materials of which are utilised for a more modern nervous system; or it may be that the correlation of spinal nerves and sympathetic are both the consequences of the formation of new organs and structures in the splanchnic area. Examined in every light, it possesses features which effectually differentiate it from the cerebro-spinal system, although it has become inextricably united with it and subservient to it.
The deep connexions of the cerebral nerves are dealt with in the section which treats of the Brain (pp. 592 to 607). Certain general points in connexion with these nerves are also touched upon in the chapter introductory to the Nervous System (p. 500). Their development is given on p. 501 et seq.
In the older accounts, the first or olfactory nerve is described as consisting of several parts: (1) a series of fine nerves, which arise from (2) the olfactory bulb. This again is connected by (3) the olfactory tract with the brain, to which it is attached by (4) two striæ or roots (Fig. 643).
The anatomy of the olfactory bulb, the olfactory tract and its roots is described elsewhere (pp. 623 to 628).
The olfactory nerve consists of about twenty separate filaments which arise in
the olfactory mucous membrane and terminate in the olfactory bulb. The fibres are non-medullated. After their origins from the olfactory cells of the olfactory region on the upper part of the nasal septum and the corresponding part of the lateral wall of the nasal cavity, the nerve fibres form fine plexuses from which the terminal filaments pass through the cribriform plates of the ethmoid on their way to the olfactory bulb. Each filament has a sheath of dura mater.
Lat. geniculate body
Occipital lobe (cut)
The second or optic nerve consists of nerve fibres which spring from the ganglion cells of the retina, and converge to the optic papilla, where they are grouped together to form the optic nerve. The nerve pierces the outer layers of the retina, the chorioid, and the sclera. It pierces the sclera 3 mm. (one-eighth of an inch) to the medial side of the posterior pole of the eyeball, and enters the orbital fat, through which it runs backwards and medially surrounded by the ocular muscles. At the posterior part of the orbit it enters the optic foramen of the sphenoid bone. through which it passes to the middle fossa of the skull, where it ends in the optic chiasma, which lies at the base of the brain, anterior to the interpeduncular area and between the right and the left anterior perforated substance.
From each of the two postero-lateral angles of the optic chiasma an optic tract sweeps round to the back of the thalamus and to the mid-brain, between the pedunculus cerebri and the hippocampal gyrus of the corresponding side, and each tract
terminates in connection with the pulvinar, the lateral geniculate body the superior brachium, and the medial geniculate body-all of the same side.
When the optic nerve reaches the optic chiasma some of its fibres pass to the optic tract of the same side and some to the optic tract of the opposite side. Therefore, each optic nerve is connected with both sides of the brain. But each optic tract, in addition to some fibres of both optic nerves, contains also fibres passing from the medial geniculate body of one side to the medial geniculate body of the opposite side.
In the orbital portion of its course the optic nerve is surrounded by sheaths of the membranes of the brain, and by a sheath of fascia bulbi, as well as by the fat and muscles; and it is crossed by the ophthalmic artery and the naso-ciliary nerve. is pierced on its inferior surface by the central artery of the retina, and as it approaches the eyeball it is surrounded by the ciliary vessels and nerves.
The third or oculo-motor nerve arises from the brain, in the region of the posterior perforated substance, by several fila radicularia (radicles) emerging from the oculo-motor sulcus, on the medial side of the cerebral peduncle, just in front of the pons (Fig. 643). Passing forwards between the posterior cerebral and
superior cerebellar arteries, the nerve pierces the dura mater beside the posterior clinoid process, in a small triangular space between the free and attached borders of the tentorium cerebelli. Beneath the dura mater the nerve courses through the
FIG. 645.-RELATIONS OF STRUCTURES IN THE CAVERNOUS SINUS AND SUPERIOR ORBITAL FISSURE.
lateral wall of the cavernous sinus, and enters the orbit through the superior orbital fissure and between the two heads of the lateral rectus muscle. As it enters the orbit it divides into upper and lower branches, separated by the nasociliary nerve.
Branches. The superior branch of the nerve supplies two muscles of the orbit the superior rectus and the levator palpebræ superioris.
The inferior branch passes forwards, and after giving branches to the medial and inferior recti, ends in the inferior oblique muscle. The short root of the ciliary ganglion arises from the terminal branch which goes to the inferior oblique muscle.
Anterior medullary velum
Brachium conjunctivum Pedunculus cerebri
FIG. 646.-DORSAL SURFACE OF THE MID-BRAIN, to show the origin of the trochlear (fourth) nerve.
Communications.-1. In the cavernous sinus the oculo-motor nerve communicates with the cavernous plexus on the internal carotid artery. 2. In the cavernous sinus it also receives a slender communication from the ophthalmic division of the trigeminal nerve. 3. The short root of the ciliary ganglion passes upwards from the branch of the nerve which supplies the inferior oblique muscle.
The fourth or trochlear nerve emerges from the dorsal surface of the midbrain. It arises at the side of the frenulum veli from the anterior end of the anterior medullary velum, just behind the corpora quadrigemina. It is extremely slender, and of considerable length. Passing round the cerebral peduncle, the nerve appears at the base of the brain behind the optic tract, in the interval between the cerebral peduncle and the temporal lobe of the brain. Continued forwards to the base of the skull, it pierces the free border of the tentorium cerebelli, postero-lateral to the oculo-motor nerve, and proceeds forwards in the lateral wall of the cavernous sinus, to the superior orbital fissure, lying between the oculo
motor nerve and the ophthalmic division of the trigeminal nerve. It enters the orbit above the muscles of the eyeball, and terminates in the orbital (superior) surface of the superior oblique muscle.
Communications. In the cavernous sinus the nerve receives (1) a communicating
FIG. 647. THE BASE OF THE SKULL, to show the dura mater, sinuses, arteries, and nerves.
branch from the cavernous or carotid plexus on the internal carotid artery, and (2) a slender filament from the ophthalmic division of the trigeminal nerve.
The fifth or trigeminal nerve arises from the inferior surface of the pons in its lateral part by two roots, a large sensory root and a small motor root (Fig. 643, p. 768). The two roots proceed forwards in the posterior fossa of the skull, and piercing the dura mater beneath the attachment of the tentorium cerebelli to the superior angle of the petrous part of the temporal bone, enter a cavity in the dura mater (cavum Meckelii) over the apex of the petrous bone.