SPINAL MARROW. 145 
which are parallel and contiguous. It may be considered 
as of a firmer consistence than the cerebrum, and contain- 
ing a greater proportion of cineritious matter. When di- 
vided vertically, the medullary matter appears disposed 
throughout the cineritious, like the branches of a tree. 
This has given rise to the appellation Arbor vita. The 
cerebellum is divided longitudinally into two lobes or he- 
mispheres, which are united with each other by means of a 
medullary part, termed the vermiform process, or third lobe 
of the brain. On the inferior surface, where it rests upon 
the commencement of the spinal marrow, there is a cavity, 
termed the fourth ventricle of the brain. It communicates 
with the third ventricle, by a passage which has been term- 
ed the Aqueduct of Silvius. In the bottom of the fourth ven- 
tricle, there is an angular impression, bearing a resemblance 
to a writing pen, and denominated Calamus scriptorius. 
2. The spINAL MARROW may either be considered as 
taking its rise from the brain, or as terminating in its sub- 
stance. If we view it as originating from the brain, it con- 
sists of four cords, which are termed Crura, two of which 
proceed from the cerebrum, and two from the cerebellum. 
Those from the cerebrum arise between the anterior and 
middle lobes. The body formed by the union of these 
four crura, when contained within the cavity of the skull, 
is called Medulla oblongata. It is separated from the 
mass of the cerebrum by a medullary sheath, striated 
transversely, termed Pons Varolit or Tuber annulare, and 
is marked with a longitudinal furrow in the middle, and one 
on each side. Within each lateral furrow, there is a slight 
eminence, denominated Corpus olivarium ; and, between 
this and the middle line, there are some longitudinal fibres, 
termed Corpora pyramidalia. In its progress downwards, 
in the canal formed in the vertebrae for its reception, it 1s 
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