, PHYSIOLOGICAL ANATOMY OF THE SPINAL CORD. 669 



pressure in the varying condition of the blood-vessels, accurately filling the space be- 

 tween the centres and the bony cavities in which they are contained. That the blood- 

 vessels of the cerebro-spinal axis are subject to variations in tension, is readily shown by 

 introducing a canula into the subarachnoid space, when the jet of fluid discharged will 

 be increased with every violent muscular effort. The pressure of the fluid, in this in- 

 stance, could only be affected through the blood-vessels. 



Physiological Anatomy of the Spinal Cord. 



The spinal cord, with its membranes, the roots of the spinal nerves, and the sur- 

 rounding liquid, occupies the spinal canal and is continuous with the encephalon. Its 

 length is from fifteen to eighteen inches, and its weight is about an ounce and a half. 

 Its form is cylindrical, being slightly flattened in certain portions. It extends from the 

 foramen magnum to the first lumbar vertebra. It presents, at the origin of the brachial 

 nerves, an elongated enlargement, and a corresponding enlargement at the origin of the 

 nerves which supply the lower extremities. It terminates below in a slender, gray fila- 

 ment, called the filum termiuale. The sacral and coccygeal nerves, after their origin 

 from the lower portion of the cord, pass downward to emerge by the sacral foramina, 

 and they form what is known as the cauda equina. 



The substance of the cord is formed of white and gray matter, the white matter 

 being external. The proportion of white matter to the gray is greatest in the cervical 

 region. This fact is important in studying the course of the fibres and in view of the 

 functions of the cord as a conductor. The inferior, pointed termination of the cord con- 

 sists entirely of gray matter. 



The cord is marked by an anterior and a posterior medium fissure, and by imperfect 

 and somewhat indistinct anterior and posterior lateral grooves, from which latter arise the 

 anterior and the posterior roots of the spinal nerves. The posterior lateral groove is 

 tolerably well marked, but there is no distinct line at the origin of the anterior roots. 

 The anterior median fissure, or sulcus, is perfectly distinct. It penetrates the anterior 

 portion of the cord in the median line for about one-third of its thickness and receives a 

 highly vascular fold of the pia mater. It extends to the anterior white commissure. The 

 posterior fissure is not so distinct as the anterior, and it is not lined throughout by a fold 

 of the pia mater, but is filled with connective tissue and blood-vessels, which form a sep- 

 tum posteriorly, between the lateral halves of the cord. The posterior median fissure, so 

 called, extends nearly to the centre of the cord, as far as the posterior gray commissure. 



Physiologically and anatomically, the cord is divided into two lateral halves ; but the 

 division of each half into columns is not so distinct. Anatomists generally regard a half 

 of the cord as consisting of three columns : The anterior column is bounded by the 

 anterior fissure and the origin of the anterior roots of the spinal nerves ; the lateral col- 

 umn is included between the anterior and the posterior roots of the nerves ; the poste- 

 rior column is bounded by the line of origin of the posterior roots and by the posterior 

 fissure. Some anatomists include the lateral with the anterior column, under the name of 

 the antero-lateral column, taking in about two-thirds of the cord. Next the posterior 

 median fissure, is a narrow band, marked by a faint line, which is sometimes called the 

 posterior median column. 



The arrangement of the white and the gray matter in the cord is seen in a transverse 

 section. The gray substance is in the form of a letter H, presenting two anterior and 

 two posterior cornua connected by what is called the gray commissure. The anterior 

 cornua are the shorter and broader, and they do not reach to the surface of the cord. 

 The posterior cornua are larger and narrow, and they extend nearly to the surface, at 

 the point of origin of the posterior roots of the spinal nerves. In the centre of the gray 

 commissure, is a very narrow canal, lined by cells of ciliated epithelium, called the 

 central canal. This is in communication above with the fourth ventricle, and it extends 



