160 OUTLINES OF EQUINE ANATOMY. 



respectively. Tlie corpora pyramidalia inferior are sepa- 

 rated from each, other by the inferior longitudinal fissure, 

 which, however, is slightly obscured by the decussation or 

 passage to the opposite side of the medulla of the motor 

 fibres of the corpora restiformia, the lateral tracts of the 

 medulla oblongata continuous with those of the spinal cord. 

 These divide anteriorly, one branch runs to the cerebellum 

 forming its posterior crus, the other passes forwards to the 

 cerebrum and unites with fibres from the pyramids forming 

 the crura cerebri. These restiform bodies are separated 

 from the inferior pyramids by the corpora olivaria, small 

 round bodies which internally present an irregular grey 

 nodule, corpus dentatum. Their upper surface forms the 

 floor of the fourth ventricle, and on either side superiorly 

 is situated a small corpus pyramidalium superior. The two 

 sides of the medulla oblongata are connected together by a 

 commissure formed of grey and white matter. 



The base of the brain presents numerous and diverse 

 structures. At the anterior part centrally is the superior 

 longitudinal fissure, and on either side of this is an olfac- 

 tory bulb. This is a rounded mass of grey matter situated 

 in the olfactory sinus, from which grey fibres run forwards 

 through foramina, in the cribriform plates. This is 

 attached to the under j^art of a tract of white matter 

 which runs backwards and becomes narrower, terminating 

 in two parts which separate at an angle enclosing a trian- 

 gular space, locus perforatus anticus, or substantia perforata, 

 the out3r surface of the corpus striatum. These tracts of 

 matter contain a cavity lined by serous membrane, which 

 in some subjects is said to have been found continuous 

 with the lateral ventricle. From the lateral parts of the 

 thalamus opticus and from the corpora quadrigemina 

 tracts of white matter run round the lateral parts of the 

 crura cerebri. At the base of the brain they unite, and 

 the fibres from this union form two nervous cords, the optic 

 nerves which pass to the eye. This is the optic decussa- 

 tion, the fibres from one side pass— to the nerve of the 

 opposite side, to the nerve of the same side, to the other 

 optic tract ; fibres also run from one optic nerve to the 

 other. From the postero-lateral parts of the optic decus- 

 sation the crura cerebri run to the anterior margin of the 

 pons Varolii. They are broad bands of white matter, and 

 from their under surface the third pair of nerves, motores 



