SPINAL MA1UIOW. 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. Wlien 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 vitce. 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 ofSilvius. 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 Varolii 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 puramidaMa. In its progress downwards, 

 in the canal formed in the vertebrae for its reception, it is 

 VOL. i. K 



