678 NERVOUS SYSTEM OF THE HORSE 



The sensory root (Portio major) extends forward through a notch on the lower 

 part of the petrosal crest and widens out to join the semilunar ganglion. 



The semilunar (or Gasserian) ganglion (Ganglion semilunare) is a crescent- 

 shaped mass of nerve-fibers and cells which overlies the antero-external part of 

 the foramen lacerum basis cranii, and is partly embedded in the dense fibrous tissue 

 which occupies the foramen except where vessels and nerves pass through. Its 

 long axis, which is about an inch (2.5 cm.) in length, is directed forward and inward 

 and its convex anterior face gives rise to the ophthalmic, the maxillary, and the 

 sensory part of the mandibular division of the nerve. The surface of the ganglion 

 is irregularly striated. It is connected by filaments with the adjacent carotid 

 plexus of the sympathetic, and sends delicate twigs to the dura mater. 



The fibers of the sensory root arise from the ganghon as axones of the ganghon cells, and the 

 fibers of the nerves which extend peripherally from the ganglion are dendrites of the cells. The 

 sensory root-fibers enter the tegmentum of the pons and divide into anterior and posterior branches, 

 which terminate about the cells of the sensory nucleus of termination of the trigeminus. This 

 nucleus extends from the pons to the sixth cervical segment of the spinal cord (Dexler). The 

 posterior branches of the fibers are collected into a compact bundle, the spinal tract or root of the 

 trigeminus, which lies lateral to the substantia gelatinosa in the medulla. The central connections 

 of the sensory part of the trigeminus are very extensive. The most important paths are: (1) 

 Axones of cells of the sensory nucleus and the substantia gelatinosa pass chiefly as arcuate fibers 

 across the raphe to the thalamus, whence impulses are transmitted by thalamo-cortical fibers 

 to the cerebral cortex. In ungulates a distinct tract extends from the anterior part of the sensory 

 nucleus to the thalamus of the same side (Wallenberg). It is probable that collaterals of the 

 arcuate fibers go to the motor nuclei of the fifth, seventh, ninth, and tenth cranial nerves. (2) 

 Axones of cells of the sensory nucleus enter the posterior cerebellar peduncle of the same side 

 and reach the cerebellar cortex. (3) Collaterals are distributed to the nuclei of origin of the 

 hypoglossal and of the motor part of the trigeminal and facial nerves. 



The motor root (Portio minor) extends forward beneath the sensory root and 

 the semilunar ganglion and is incorporated with the mandibular division of the 

 nerve. Its fibers arise chiefly from the so-called masticatory nucleus, which is 

 situated in the pons near the inner face of the sensory nucleus; a few of these fibers 

 come from the nucleus of the opposite side and cross in the raphe. Other fibers, 

 which constitute the mesencephalic root, arise from cells in the outer part of the 

 central gray matter of the mid-l)rain. 



It is evident from the foregoing statements that the trigeminus has essentially 

 the same arrangement as a typical spinal nerve. It divides into three branches. 



I. The ophthalmic nerve (N. ophthalmicus) (Figs. 438, 439, 440) is purely 

 sensory and is the smallest of the three branches of the trigeminus. It arises from 

 the inner part of the front of the semilunar ganglion, passes forward along the outer 

 side of the cavernous sinus, and is blended with the maxillary nerve for some dis- 

 tance. It enters the foramen lacerum orbitale with the third and sixth nerves and 

 divides into three branches. 



1. The lacrimal nerve (N. lacrimalis) runs forward on the rectus superior and 

 the levator palpebral superioris and ramifies chiefly in the lacrimal gland and the 

 upper eyelid. A branch (Ramus zygomatico-temporalis) exchanges twigs with the 

 zygomatic branch of the maxillary nerve, perforates the periorbita, and emerges 

 from the orbital fossa beliind the supraorl^ital process; it forms a plexus with 

 branches of the auriculo-palpebral and frontal nerves, and ramifies in the skin of 

 the temporal region. 



2. The frontal nerve (N. frontalis), also termed the supraorbital, runs forward 

 almost parallel with the superior oblique muscle, at first within, then outside of, the 

 periorbita. It passes through the supraorl^ital foramen with the artery of like 

 name and ramifies in the skin of the forehead and upper eyelid, forming a plexus 

 with the lacrimal and auriculo-palpebral nerves. It divides into three branches. 



3. The naso-ciliary nerve (N. naso-ciliaris), also termed the palpebro-nasal, 

 runs forward along the inn(>r side of the optic nerve between parts of the retractor 

 muscle and divides into two branches. Of these the ethmoidal nerve (N. ethmoi- 



