THE BRAIN 



771 



The ninth or glosso-pharyngeal, the tenth or vagus, and the eleventh or spinal 

 accessory nerves are connected by a Unear series of roots with the lateral aspect of 

 the ventral surface of the medulla. The spinal part of the accessory nerve comes 

 forward along the edge of the medulla to join its medullary root. 



The twelfth or hypoglossal nerve arises from the posterior part of the medulla 

 along the lateral edge of the pyramid. 



The parts that are visible when the brain is viewed from above are the cerebral 

 hemispheres, the cerebellum, and part of the medulla oblongata. The cerebral 

 hemispheres (Hemispharia cerebri) form an ovoid mass, and are separated from 

 each other by the median longitudinal fissure (Fissura longitudinalis cerebri), 



Longitudinal fissure 



Trigciiiinai 

 nerve 



Sulcus rhinalis'. 



Optic nerve ^ 



Optic chiasma^ 



Tuber cinereum-. 



Piriforin lobe -. 



Pituitary body- 



Oculomotor nerve - 



sensory root ■^ 



motor root — 

 Cerebellum -_ _ 



Tuberculum faciale 



Glosso-pharyngeal nerve 



Vagus nerve 



Accessory nerve {medullary root) 



Accessory nerve {spinal root) 



Hypoglossal nerve 



Olfactory bulb 



^^- Olfactory tract 



-'Medial stria 



Lateral stria 



— Trigonum olfactorium 

 "Fossa lateralis 



- Cerebral peduncle 

 Tractus ped. transversus 

 '_ hiterpeduncular fossa 



Pons 



Corpus trapezoideum 



Pyramid 

 Medulla oblongata 



"" — Chorioid plexus of fourth 



ventricle 

 — Median fissure and de- 

 cussation of pyramids 



— Spinal cord 



Fig. 630. — Line Drawing of Base of Brain of Horse. (Key to Fig. 629.) 

 VI, Abducent nerve; VII, facial nerve; VIII, acoustic nerve. 



in which the falx cerebri is situated. Their surfaces are marked by thick ridges, 

 the g3rri cerebri, separated by sulci. The upturned ends of the olfactory bulbs 

 are seen in front of the frontal poles of the hemispheres. The occipital poles of 

 the hemispheres overlie the anterior part of the cerebellum, from which they are 

 separated by the transverse fissxxre (Fissura transversa cerebri) and the tentorium 

 cerebelli contained in it. The cerebellum is a much smaller rounded mass which con- 

 ceals the greater part of the medulla oblongata. Its surface is divided into a middle 

 lobe, the vermis cerebelli, and two lateral hemispheres (Hemisphaeria cerebelli). It 

 is marked by numerous gyri and narrow sulci which have in general a transverse direc- 

 tion. The posterior third of the medulla oblongata is not covered by the cerebellum. 



The brain is developed from the expanded cephalic part of the neural tube of the embryo. 

 The process comprises a series of thickenings, flexures, and unequal growth and expansion of 



