666 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



(This ligament, or membrana dentata, serves to maintain the position of 

 the spinal cord in its hydrostatic bed, and to prevent the nerves proceeding 

 from it being dragged during flexion of the spine.) 



CRANIAL, OR ENCEPHALIC PIA MATER. The vascular element pre- 

 dominates in this portion of the internal meninge. 



This membrane sends scarcely any prolongations to the dura mater, 

 except at the medulla oblongata, though it projects remarkably large ones 

 into the cerebral mass and the sides of the cerebellum. The description of 

 the velum interposition, and the cerebral and cerebellar plexus choroides, belongs 

 to the encephalon. 



(The pia mater is extremely vascular on the surface of the cerebrum, and 

 forms remarkable anastomosing loops in the intermediate spaces of the 

 convolutions, which chiefly supply the grey substance. It is the nutrient 

 membrane of the brain and spinal cord. Its nerves accompany its arterial 

 branches, and are minute filaments from the sympathetic.) 



DIFFERENTIAL CHARACTEES IN THE ENVELOPING AND PROTECTIVE PARTS OF THE CEREBRO- 

 SPINAL AXIS IN OTHER THAN SOLIPED ANIMALS. 



The oony canal that protects the spinal cord and brain does not present any note- 

 worthy differences in the domesticated animals, and the subject has been already 

 sufficiently studied in the osteology of the head and vertebral column. 



With regard to the meninges, their number and general disposition are the same in 

 all the species. 



COMPARISON OF THE ENVELOPING AND PROTECTIVE PARTS OF THE CEREBRO-SPINAL 

 AXIS OF MAN WITH THOSE OF ANIMALS. 



There is nothing particular to be said respecting the cranial cavity and spinal canal, 

 nor yet the arachnoid and pia mater. The dura mater offers the folds described in 

 Solipeds, and, in addition, &falx cerebellum, that extends from the tentorium of the same 

 name to near the foramen magnum. The meningeal granulations, or Pacchionian glands, 

 are nearly constant in aged individuals, and their volume is sometimes so considerable, 

 that by compression they thin away, and even perforate, the cranium at corresponding 

 points. 



CHAPTEE II. 



THE SPINAL CORD. 



Preparation. Isolate the cranium and vertebral column from all the other parts of the 

 body; open the spinal canal and the cranial cavity by their superior surface, as in 

 figure 316, by raising with a chisel (or rogne-pied, the farrier's " toe-knife ") and hammer, 

 the roof of the skull and annular portion of all the vertebrae. The organ may then be 

 studied in situ in its bony case, and surrounded by its membranes ; afterwards extract 

 the whole cerebro-spinal axis inclosed in the dura mater, and open up the latter along 

 the course of the cord, so as to completely expose that portion of the nervous system. 



(The saw and farrier's pincers, or spine ratchet, will be found useful auxiliaries in the 

 tedious and delicate operation of exposing the brain and cord ; and particularly in lay- 

 ing open the cranial cavity. An easy mode of obtaining access to the spinal canal and 

 its contents, is to saw through the laminae of the vertebrae on each side, at the roots of 

 the transverse processes, and raise the arches with the chisel or toe-knife.) 



EXTERNAL CONFORMATION OF THE SPINAL CORD. 



General view. The spinal cord is that portion of the nervous centres which 

 occupies the spinal canal. It is a thick, white, and irregularly cylindrical 

 cord, commencing at the occipital foramen, where it continues the medulla 

 oblongata, terminating in a point at the upper third of the sacral canal, or a 



