THE CRANIAL NERVES. 129 



the mass in laminated shape, and is traversed by curved furrows, between the 

 laminse. A vertical section gives an appearance called the Arbor Vita. 



Corpus Dentatum, is a capsule of gray matter in the centre of the white 

 and is open anteriorly. It is sometimes called the ganglion of the cere- 

 bellum. 



THE SPINAL CORD. 



Describe the Spinal Cord. It is that part of the cerebro-spinal axis which 

 is situated in the spinal canal. Its length is about 16 inches, terminating at 

 the lower border of the ist lumbar vertebra in the Cauda Equina. It is cylin- 

 drical in general form, with 2 enlargements, one in the cervical region, the 

 other in the lumbar. It is composed of gray and white matter, the gray being 

 inside (instead of outside, as in the cerebrum), and arranged so as to present 

 a crescentic appearance in horizontal section, joined by a transverse commis- 

 sure, and forming, by their extremities, the Anterior and Posterior Horns, from 

 which regions respectively the anterior and posterior roots of the spinal nerves 

 have their apparent origin. 



Membranes of the cord are 3, as in the brain, dura mater, arachnoid, 

 and pia mater. The Dura Mater is not adherent to the spinal column, 

 but is connected thereto by fibrous tissue. The Arachnoid is arranged as 

 on the brain, its sub-arachnoid space being filled with its fluid, for the 

 protection of the cord. The Pia Mafer has a fibrous band on each side, 

 the ligamentum denticulatum, connecting it to the dura mater by some 20 

 serrations. 



Fissures, number 8, the anterior and posterior median, the anterior and 

 posterior lateral on each side, and the two posterior intermediary in the 

 cervical region. 



Columns, are 8 in number, 4 on each side of the cord, an anterior, lateral, 

 posterior, and posterior median. The Anterior is continuous with the 

 anterior pyramid of the medulla; the Lateral, with the lateral column of 

 the medulla ; the Posterior, with the restiform body ; the Posterior-median, 

 with the posterior pyramid. 



Central Canal, or Ventricle of the Cord, extends through its entire length in 

 the fcetus and in some adults, but is usually closed except for one-half inch 

 below its orifice in the floor of the 4th ventricle. 



THE CRANIAL NERVES. 



Describe each Cranial Nerve, stating its function, apparent and deep 

 origin, foramen of exit, principal branches and distribution. There are 

 12 pairs of cranial nerves (9 according to Willis), of -which those from the 6th 

 to the 1 2th inclusive have their deep origin wholly or in part from the floor of 

 die 4th ventricle. They are the 



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