580 THE NERVOUS SYSTEM 



peripheral tissues or in other parts of the nervous system. For example, the 

 vagus center regulates the activity of the heart muscle by its power to decrease 

 or inhibit cardiac contractions. This center, we have already found, is in 

 constant tonic activity; that is to say, in constant regulative control of the 

 heart. The cardiac augmentory center, on the other hand, produces just the 

 opposite effect, increasing the activity of the cardiac muscle. What is true 

 for the heart is likewise true in general for other tissues of the body. The 

 numerous nerve centers in the central nervous system are brought into cor- 

 relation through an exceedingly complex system of communicating fibers. 

 The cerebro-spinal axis may in fact be regarded as a segmental chain of nerve 

 centers, the complexity increasing from the cord toward the brain, and the 

 coordinating control culminating in the cerebral cortex. 



THE STRUCTURE AND FUNCTION OF THE 

 SPINAL CORD. 



STRUCTURE. 



The spinal cord is a cylindrical column of nerve-substance connected 

 above with the brain through the medium of the bulb, and terminating below 

 in a slender filament of nerve substance, the filum terminale, which lies in the 

 midst of the roots of the many nerves forming the cauda equina. 



General Features. The cord is composed of nerve fibers and nerve 

 cells. The former are situated externally and constitute the chief portion 

 of the cord, while the latter occupy its central or axial portion and are so 

 disposed that on the surface of a transverse section of the cord two somewhat 

 crescentic grayish masses connected by a narrower portion or isthmus ap- 

 pear, figure 358. Passing through the center of the cord in a longitudinal 

 direction is a minute canal, the central canal, which is continued through 

 the whole length of the cord, opening above into the space at the back of 

 the medulla oblongata and pons Varolii called the fourth ventricle. The 

 canal is lined by a layer of columnar ciliated epithelium. 



The spinal cord consists of exactly symmetrical halves, separated an- 

 teriorly and posteriorly by vertical fissures (the posterior fissure being deeper 

 but less wide and distinct than the anterior), and united in the middle by 

 nervous matter which forms the commissures. The central part, which 

 contains the central canal, is known as the gray commissure, and is bounded 

 by the anterior white commissure in front. Each half of the spinal cord is 

 marked on the sides (obscurely at the lower part, but distinctly above) by 

 two longitudinal furrows, which divide it into three portions, funiculi, or tracts 

 an anterior, lateral, and posterior. From the groove between the anterior 

 and lateral funiculi spring the anterior roots of the spinal nerves; and just 



