THE MEMBRANES OF THE BRAIN AND CORD. 235 



pathy with each other; and even the vegetative organs, though pro- 

 vided with their own nervous apparatus, may be so affected through 

 the irritation of distant parts, that a strong light applied to the eye, 

 will cause vomiting, and an emotion or desire will create palpitation 

 of the heart. So widely diffused, indeed, are the phenomena of sen- 

 sation, regulated motion, and psychical action in the body, that it is 

 easier, instead of describing these functions in strictly physiological 

 order, to treat of the functions of the several parts of the nervous sys- 

 tem. To prepare the way for such an account, we must first give a 

 description of this system. 



THE STRUCTURE OF THE NERVOUS SYSTEM. 



The nervous system in man, includes the cerebro- spinal nervous sys- 

 tem, and the sympathetic nervous system. The former consists of a 

 central part, composed of the brain and spinal cord, often designated 

 together, the cerebro-spinal axis, and of a peripheral part, composed 

 of nerves which spring from the brain and cord, and are named the 

 cranial and spinal nerves. The sympathetic nervous system consists 

 of numerous ganglia, connecting nerve cords, and branches or nerves : 

 it is joined, by numerous short cords, with the cerebro-spinal system. 



The Cerebro- Spinal Nervous System. 



The protected position of the brain and spinal cord, within the cavity 

 of the cranium and the spinal canal, as well as the mode in which the 

 nerves escape from those cavities, have been already described (pp. 

 27, 30, Figs. 9 and 1*2); and the position of the sympathetic nerves in 

 the thorax and abdomen, is mentioned at pp. 35, 36, and is further 

 illustrated in Fig. 62. 



The three protective membranes which cover the brain and spinal 

 cord, viz., the fibrous membrane or dura mater, the serous membrane 

 or arachnoid, and the vascular membrane or pia mater, have also been 

 mentioned. The dura mater protects the brain and cord, forms sup- 

 porting partitions between the right and left halves of the cerebrum, 

 and between the cerebrum and cerebellum, and furnishes sheaths to the 

 several cranial and spinal nerves; it likewise assists in forming venous 

 channels or sinuses for the conveyance of the blood returning from 

 the brain; in the cranium, it acts as an internal periosteum. The 

 arachnoid, like other serous membranes, is a doubled membrane form- 

 ing a closed sac, the purpose of which is to facilitate such motions of 

 the brain and cord, as are inevitable in changes of their position, or 

 of the state of their circulation. The spinal cord is specially supported 

 by a long duplicature of the arachnoid, with fibrous tissue in it, on 

 each side, named the ligamentum denticulatum, Fig. 60, t. Between 

 the arachnoid and the innermost membrane, or pia mater of the brain 

 and cord, are certain spaces filled with fluid, the subarachnoid fluid, 

 which is said to support the great nervous centres as in a hydrostatic 

 apparatus. Its usual quantity is about two ounces, but this varies in 

 certain circumstances; in cases of extravasation of blood in the head, 

 it is all absorbed ; whereas, in wasting of the brain, it is increased to 



