666 



NERVOUS SYSTEM OF THE HORSE 



It is glandular in character and there is good ground for the view that it is an organ of internal 

 secretion. Besides the chief cells which stain lightly, it contains large deeply staining chromophile 

 cells. It arises as an outgrowth from the primitive mouth cavity. The cerebral (or posterior) 

 lobe is pale and is connected with the infundibulum so as to form a rather flask-shaped arrange- 

 ment. It is almost entirely inclosed by the glandular part. It arises as an outgrowtli from the 

 j)rimitive diencephalon, but loses most of its earlier nervous character. 



The optic chiasm and tracts form the anterior boundary of the interpeduncular 

 space. The optic chiasm or commissure (Chiasma opticum) is formed Idj the 

 convergence of the optic nerves and the crossing of the major part of the fibers of 

 the nerve of one side to the tract of the opposite side. From the chiasm each 

 optic tract (Tractus opticus) curves over the cerebral peduncle outward, back- 



Sulcus rhinal 

 Ventral horn of 

 lateral ventricle 



',' Optic tract 



I intta ' 



thalami 



Fig. 506. — Cross-section of Brain of Horse, Natural Size. 

 Section passes through posterior part of third ventricle and is viewed from behind. 1 ., Longitudinal fissure; 

 3, hippocampus; i?', fimbria; S, .septum peUucidum; 4. lateral ventricle; 5, thalamus; 6, habenula; 7, third ven- 

 tricle; S, cerebral peduncle; S' , hypothalamus; 9, mammillary body; 10, pituitary liody; 11, pyriform lobe; 

 J2, ventral enrl of hi|>pocampus; 13, amygdaloid nucleus. Between the upper parts of the taenia- thalami is the 

 chorioid plexus of the third ventricle, and above C.iis are the internal cerebral veins 



ward, and ujiward to the posterior part of the thalamus and the internal geniculate 

 body; some fibers reach the anterior (luadrigeminal body. 



All the fibers in the chiasm are not derived from the optic nerves. The posterior part 

 contains fibers which pass from one tract to the other and are connected with the internal genicu- 

 late bodies; this bundle is called Gudden's commissure (Commissura inferior). Above it is 

 Meynert's commissure (Commissura superior), the fibers of which enter the subthalamic body. 



The third ventricle (Ventriculus tertius) is the narrow annular space between 

 the thalami. It comnmnicates by means of the acjueduct with the fourth ven- 

 tricle behind, and in front it is continuous with the lateral ventricle on each side 

 through the interventricular foramen. Its floor is formed liy the structures of the 

 interpeduncular space and to a small extent by the tegmentum of the cerebral 

 peduncles. The roof is formed in the strict sense only by the ependyma, above 

 which is a fold of ])ia muter, termed the tela chorioidea of the third ventricle or 

 velum interpositum. The roof is invaginated by two delicate chorioid plexuses 



