HEAD AND NECK OF THE HORSE 181 



Numerous small olfactory nerves leave the olfactory bulb and 

 immediately enter the foramina of the ethmoid bone to gain the cavity 

 of the nose. They are generally left behind when the brain is removed. 

 The optic nerve is a large rounded cord connected with the optic 

 chiasma, through which its fibres are continued into the optic tracts. 

 The oculomotor nerve has origin from the cerebral peduncle about the 

 middle of its length and towards its medial border. The origin of the 

 trochlear nerve from the anterior medullary velum cannot be seen at 

 present ; but the nerve itself will be found curving round the lateral 

 border of the cerebral peduncle, in the cleft between this and the 

 cerebrum and cerebellum. The trochlear is the smallest of the cerebral 

 nerves. The trigeminal ^ nerve is the largest of the cerebral group and 

 arises by two roots from the lateral part of the pons. The larger root 

 (portio major) is sensory in function, while the much smaller medial 

 root (portio minor) is motor. The abducent nerve has its origin from 

 the most anterior part of the groove that forms the lateral boundary of 

 the pyramid of the medulla oblongata. The facial and acoustic nerves 

 arise together from the lateral extremity of the corpus trapezoideum ; 

 the facial being the more medial and slightly the smaller. The 

 glossopharyngeal, vagus and accessory nerves are formed by the union 

 of a series of nerve rootlets that leave the lateral border of the ventral 

 surface of the medulla oblongata. The accessory nerve, in addition to 

 its fine medullary roots, contains fibres derived from the first iive 

 cervical segments of the spinal cord. These unite into a single cord 

 that enters the cranium by the foramen magnum and joins the 

 medullary roots to form the complete nerve. The hypoglossal nerve 

 arises by several rootlets from the posterior part of groove lateral to the 

 pyramid. 



Dissection. — As a preliminary to the examination of the surface of the 

 cerebral hemispheres, they must be carefully denuded of the covering 

 formed by the pia mater and arachnoid. Shice the whole of their surface 

 cannot be examined without tlie separation of the two hemispheres, two 

 brains should be procured, if possible. One of these should be left 

 intact ; while the cerebellum, medulla oblongata and pons of the other 

 should be separated from the cerebrum by a transverse incision made just 

 in front of the pons. The two hemispheres of this specimen may now be 

 isolated by an incision in the plane of the great longitudinal fissure. 



The cerebral hemispheres (Hemisphseria cerebri). — The two 

 cerebral hemispheres form the greater part of the bulk of the brain. 

 Each is flattened laterally in a certain degree, and so possesses surfaces 

 that are generally described as three in number — dorso-lateral, medial 

 and ventral — though there is no clear line of distinction between the 



o 



1 Trigeminus [L.], three born at a birth, a triplet. (From the three primary 

 divisions of the nerve.) 



