210 ANATOMY FOE NUKSES. [CHAP. XVIII. 



nerves. The nose has also one special nerve, the olfactory, and 

 is more abundantly supplied than the ear, with motor and sen- 

 sitive fibres from other nerves. The tongue has two special 

 branch nerves of taste, the lingual, a branch of the fifth, and 

 the glossal, a branch of the ninth: it has also its own motor 

 nerve, the hypoglossal. 



The spinal cord and spinal nerves. The spinal cord is a 

 column of gray and white soft substance, extending from the 

 top of the spinal canal, where it is continuous with 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. 



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. The dura mater is not attached to the walls of the spinal 

 canal, being separated from them by a certain quantity of areo- 

 lar and adipose tissue and a network of veins. Therefore, 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 downwards, 

 with the exception of presenting two enlargements in the 

 cervical and dorsal regions. It is usually from sixteen to 

 seventeen inches long, and has an average diameter of three- 

 fourths of an inch. 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 conse- 

 quence of the presence of these fissures, only a narrow bridge 

 of the substance of the cord connects its two halves, and this 

 bridge is traversed throughout its entire length by a minute cen- 

 tral canal, the canalis centralis. On making a transverse sec- 

 tion of the spinal cord, the gray matter is seen to be arranged in 

 each half in the form of a half-moon or crescent, with one end 

 bigger than the other, and with the concave side turned out- 

 wards. The convex sides of the gray matter in each half 

 approach one another, and are joined by the isthmus or bridge 

 which contains the central canal. The tips of each crescent 

 are called its horns or cornua, the front or anterior cornua 

 being thicker and larger than the posterior. The white mat- 



