672 



NERVOUS SYSTEM OF THE HORSE 



The pyriform lobe (Lobus piriformis) is the well-marked prominence on the 

 base lateral to the optic tract and cerebral peduncle, from which it is separated by a 

 deep fissure. Its nipple-like apex lies behind the fossa transversa and covers the 

 optic tract. The external surface is marked by one or two sulci (lobi piriformis). 

 The lobe contains a cavity, the ventral horn of the lateral ventricle. 



The fibers of the olfactory tracts go to the pyriform lobe and hippocampus, the trigonum 

 olfactorium, the area parolfactoria, the subcallosal gyrus, and part of the gyrus fornicatus. The 

 central connections of the olfactory apparatus are complex and are not yet fully understood. The 

 anterior cerol)ral commissure contains fibers which pass from the olfactory bulb of one side by 

 way of the inner tracts to the bulb of the opposite side; also fibers which cross in it from the inner 

 tract of one side to the pyriform lobe of the opposite side. Many fibers pass by way of the septum 

 pellucidum, fornix, and fimbria to the hippocampus. Other fibers pass in the anterior pillar of 

 the fornix to the mammillary body and thence to the thalamus by the thalamo-mammillary 

 bundle. 



The corpus callosum is the great transverse commissure which connects the 

 two cerebral hemispheres through about half of their length. On median section 

 (Fig. 508) it is seen to be arched from before backward, white in color, and com- 



Cm 



Fig. 510. — Lateral View of Cast of Cavities of Brain of Horse. 

 Bo, Cavity of olfactory, which communicates through the canal / with a lateral ventricle; Ca, anterior 

 horn. Cm, body, Ci, ventral horn of lateral ventricle; M, isthmus which connects lateral and third ventricles 

 Ep, suprapineal recess, below which is the small infrapineal recess; Ro, ojitic recess; Rh, infundibular and pituitary 

 recess; iS', aqueduct; Fl, ridge corresponding to sulcus limitans; Rl, lateral recess, Rm, posterior recess, of fourth 

 ventricle; Cc, beginning of central canal of spinal cord. (Dexler.) 



posed substantially of transverse fibers. The middle part or truncus (Truncus 

 corporis callosi) slopes downward and forward and is thinner than the ends. The 

 anterior thickened end, the genu, bends downward and backward and thins out to 

 form the rostrum ; the latter is continuous with the lamina terminalis. The pos- 

 terior end, the splenium, also thick, lies at a considerably higher level than the 

 genu. The upper surface is convex in its length, concave transversely. Its central 

 part forms the floor of the longitudinal fissure. It is covered by a thin layer of 

 gray matter (Induscnim griseum), in which are strands of longitudinal fibers (Striae 

 longitudinales) ; the latter are arranged in median and lateral bundles (Str'a 

 medialis, striae laterales).^ The lower surface has the reverse configuration, and 

 forms the roof of the lateral ventricles. Along the median line the septum 

 pellucidum is attached to it. The fibers of the corpus callosum (with the exception 

 of the longitudinal stria^) run transversely and spread out laterally in all directions in 

 the central white matter of the hemispheres to the cortex, forming the radiation of 

 the corpus callosum (Radiatio corporis callosi). 



' It is generally believed that these constitute an olfactory path. 



