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 outgrowth from the 

 primitive diencephalon, but loses most of its earlier nervous character. 



The optic chiasm and tracts form the anterior l)ounclary of the interpeduncular 

 space. The optic chiasm or commissure (Chiasma opticum) is formed l^y 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 rhinalis 

 Ventral horn of 

 lateral ventricle 



Optic tract 



Fii;. .506. — Cross-section of Brain of Horsi:, Natural Size. 

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

 2, hippocampus; 2' , fimbria; S, septum pellucidum; 4. lateral ventricle; 5, thalamus; 6, habenula; 7, third ven- 

 tricle; 8, cerebral peduncle; 8', hypothalamus; 9, mammillary body; 10, pituitary body; //, pyriform lobe; 

 J3, ventral end of hippocampus; IS, amygdaloid nucleus. Between the upper parts of the tirnia- thalami is the 

 chorioid ple.xus of the third ventricle, and above this are the internal cerebral veins. 



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

 body; some fibers reach the anterior quadrigeminal l)ody. 



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

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

 late bodies; this l)undle is called (ludden's commissure (Commissura inferior). Above it is 

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



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

 the thalami. It communicates by means of the aqueduct 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 by the structures of the 

 interpeduncular space and to a small extent l)y the tegmentimi of the cereliral 

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

 which is a fold of pia mater, termed the tela chorioidea of the third ventricle or 

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



