254 THE ANATOMY OF THE HORSE. 



of the olfactory bulb, and pass through the foramina of the cribriform 

 plate to reach the summit of the nasal chamber. They are there distri- 

 buted in the olfactory division of the lining membrane of that chamber. 

 The Optic or 2nd nerve arises from the optic chiasma or commisHure, 

 and reaches the back of the orbit by passing through the optic foramen. 

 Piercing the sclerotic and choroid tunics of the eyeball, its fibres radiate 

 outwards and form one of the layers of the retina. As already seen, the 

 optic chiasma is formed by the fusion of the optic tracts, each of which 

 derives its fibres from the optic thalamus, corpora geniculata, corpora 

 quadrigemina, and decussation of the pyramids. In the optic chiasma 

 some of the fibres of each tract cross and are continued in the optic 

 nerve of the opposite side. Some of the fibres of each optic tract, it is 

 stated, cross in the chiasma and return to the brain by the opposite 

 tract, while in the same way fibres pass from the one optic nerve to the 

 other optic nerve. 



The OcuLO-AiOTOR or 3rd nerve arises from the inner side of the crus 

 cerebri by a number of bundles, the fibres of which are traceable to 

 nerve cells in the corpora quadrigemina. The nerve leaves the cranium 

 by the foramen lacerum orbitale, and reaches the orbit. 



The Pathetic, Trochlear, or 4th nerve appears to arise in the valve 

 of Vieussens (Plate 35). Some of its fibres are decussate with those of 

 the opposite nerve, and the others are traceable to nerve cells of the 

 locus cseruleus, or of the corpora quadrigemina. Emerging from the 

 valve, the nerve winds round the crus cerebri, and appears in front of 

 the pons. It leaves the cranium by the minute pathetic foramen, and 

 reaches the back of the orbit. It is the smallest of the cranial nei'ves. 



The Trifacial, Trigeminal, or 5th nerve springs out of the side of 

 the pons by two roots. The outer and larger of these is termed the 

 sensory root; and its fibres are traceable to cells of the grey matter of 

 the medula, pons, and locus cseruleus, and possibly also to the cerebellum. 

 This root near its origin expands into a large ganglion — the Gasserian 

 ganglion, beyond which it divides into three branches, viz., the ophthal- 

 mic, superior maxillary, and inferior maxillary divisions. The inner or 

 motor root of the 5th nerve is traceable to grey matter of the pons. It 

 joins the inferior maxillary division of the sensory root. The superior 

 maxillary division leaves the cranium by the foramen rotundum, the 

 ophthalmic division by the foramen lacerum orbitale, and the inferior 

 maxillary division by the forepart of the foramen lacerum basis cranii. 

 The trifacial is the largest of the cranial nerves. 



The Abducent or 6th nerve. This nerve springs from the anterior 

 part of the medulla, in line with the faint groove that limits outwardly 

 the inferior pyramid. Some of its fibres issue from the groove between 

 the pons and the medulla, while others penetrate the trapezium. The 

 fibres of the nerve are traceable to a group of nerve cells in the medulla. 



