378. ANATOMY FOR NURSES [Chap. XIX 



Vasomotor centres. — The chief vasomotor centres are situated 

 in the medulla oblongata, but there are also subsidiary centres in 

 the spinal cord. The vasomotor nerves are of two kinds, — vaso- 

 constrictor and vaso-dilator. While these nerves are always 

 considered as belongino; to the sympathetic system, it should be 

 noted that the centre that controls them is located in the medulla, 

 which is part of the central nervous system. 



SPINAL CORD 



The spinal cord is that portion of the nervous system lodged 

 within the spinal canal of the vertebral column. It consists of a 

 collection of gray and white substance, extending from the foramen 

 magnum of the skull, where it is continuous with the medulla 

 oblongata of the brain, to about the second lumbar vertebra, where 

 it tapers off into a fine thread. Before its termination it gives off 

 a number of fibres which form a tail-like expansion, called the 

 Cauda equina. 



Membranes of the cord. — Like the brain, the spinal cord is 

 protected and nourished by three membranes. These membranes 

 have the same names and practically exercise the same functions 

 as those enveloping the brain (for description of which see page 

 384). The outer membrane is not attached to the walls of the 

 spinal canal, being separated from them by a certain quantity 

 or areolar and adipose tissue, and a network of veins. 



Structure of the cord. — The spinal cord does not fit closely 

 into the spinal canal, as the brain does in the cranial cavity, 

 but is, as it were, suspended within it. It diminishes slightly in 

 size from above downward, with the exception of presenting two 

 enlargements in the cervical and lumbar regions, where the 

 nerves are given off to the arms and legs respectively. It is 

 usually from sixteen to seventeen inches (400 to 425 mm.) long, 

 and has an average diameter of three-fourths of an inch (19 mm.). 

 The spinal cord is almost completely divided into lateral halves 

 by an anterior and posterior fissure, the anterior fissure dividing 

 it in the middle line in front, and the posterior fissure in the middle 

 line behind. In consequence of the presence of these fissures, 

 only a narrow bridge of the substance of the cord connects its two 

 halves. This bridge, also called isthmus, is traversed throughout 

 its entire length by a minute central canal. On making a trans- 



