754 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



spinal cord in its hydrostatic bed, and so prevents the nerves proceeding from 

 it being dragged during flexion of the spine.) 



Cranial or Encephalic Pia Mater. — The vascular element predominates 

 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 clioroides, belongs to the brain. 



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

 forms remarkable anastomosing loops in the intermediate spaces of the con- 

 volutions, 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 Characters in the Enveloping and Protective Parts op the Cerebro- 

 spinal Axis in the Other Animals. 



The bony space tliat protects the spinal cord and brain does not present any noteworthy 

 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 nearly the same in 

 all the species. In the Sheep, however, the crauial dura mater appears to be absent, or is 

 only represented by a mere thickening in which is the middle sinus ; the falx cerebri is only 

 indicated towards its two extremities The vestige of this septum is more developed in the Goat. 



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, n^r 

 yet the arachnoid and pia mater. The dura mater has the folds described in Solipeds, and, 

 in addition, a /aZaj cerebeHi, that extends from the tentorium of the same name to near the 

 foramen magnum. The meningeal granulations, or Paccliionian glands, are nearly constant 

 in aged individuals, and their volume is sometimes so considerable, that by compression they 

 thin away, and even perforate, the cranial bones at corresponding points. 



CHAPTER IT. 

 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 Fig. 416, 

 by raising with a chisel (or rogne-pied — the farrier's '' tue-knife ") and hammer, the roof of the 

 skull and annular portion of all the vertebrae. The organ may then be studied in sitv in its 

 bony case, and surrounded by its membrane ; afterwards extract the whole spinal cord 

 enclosed 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. 



On a cord hardened by water to which a tentii part of nitric acid has been added, the 

 arrangement and distribution of the grey and white matter may be studied; the different 

 cords composing jt mny be also more or less exactly separated. 



(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 laying open 

 the cranial cavity. An easy mode of obtaining access to the spinal canal and its contents, is 

 to saw through tiie laminae of the vertebrae on each side, at the roots of the transverse processes, 

 and raise the arches with tlie 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, 



