118 EQUINE ANATOMY. 



and ail external, presenting the superior spinous pro- 

 cess and two transverse processes. In front and be- 

 hind are found two articular facets, and at their base 

 notches for the formation of the foramina of conjuga- 

 tion. Development : two principal and five secondary 

 centers. 



In the vertebral canal is found the spinal makkow. 

 It is covered by the three membranous envelopes, dura 

 mater, arachnoid and pia mater. It is a thick, white, 

 hollow mass, continuation of the medulla oblongata, 

 and ending in a point at the anterior third of the sacral 

 canal. Weighing about ten ounces, it is slightly de- 

 pressed from above below, and presents a brachial and 

 lumbar enlargements or bulbs where the nerves of the 

 fore and hind extremities are given off. Its external 

 surface shows on the superior and inferior face a deep 

 fissure, and on each side of these, two others scarcely 

 noticeable. Its internal structure shows that it is hol- 

 low, and presents in its middle a mass of grey matter 

 surrounded by white substance. They are so arranged 

 that the superior and inferior fissures of the external 

 surface are separated by a white and a grey commis- 

 sure, the last tending upwards and downwards towards 

 the surfaces of the organ prolongations or cornua. The 

 medulla receives blood from arteries of the pia mater, 

 and branches of the median spinal artery. 



