ARRANGEMENT OF THE CEREBRO-SPINAL AXIS. 667 



The dura mater of the encephalon is a dense, fibrous membrane, in two layers, com- 

 posed chiefly of inelastic tissue, which lines the cranial cavity and is adherent to the 

 bones. In certain situations, its two layers become separated and form what are known 

 as the venous sinuses. The dura mater also sends off folds or processes of its internal 

 layer. One of these passes into the longitudinal fissure and is called the falx cerebri ; 

 another lies between the cerebrum and the cerebellum and is called the tentorium ; 

 another is situated between the lateral halves of the cerebellum and is called the falx 

 cerebelli. The dura mater is closely attached to the bone at the border of the foramen 

 magnum. From this point it passes into the spinal canal and forms a loose covering for 

 the cord. In the spinal canal, this membrane is not adherent to the bones, which have, 

 like most other bones in the body, a special periosteum. At the foramina of exit of the 

 cranial and the spinal nerves, the dura mater sends out processes which envelop the 

 nerves, with the fibrous sheaths of which they soon become continuous. 



The arachnoid is an excessively delicate serous membrane, in two layers, the surfaces 

 of which are nearly in contact. The external layer lines the internal surface of the dura 

 mater. Like the other serous membranes, the arachnoid is covered with a layer of tes- 

 selated epithelium. There is a small amount of liquid between the two layers of the 

 arachnoid ; but by far the greatest quantity of liquid surrounding the cerebro-spinal 

 axis lies beneath both layers, in what is called the subaraclmoid space. This is called 

 the cerebro-spinal, or cephalo-rachidian fluid. The arachnoid does not follow the con- 

 volutions and fissures of the encephalon or the sulci of the cord, but it simply covers their 

 surfaces. Magendie pointed out a longitudinal, incomplete, cribriform, fibrous septum in 

 the cord, passing from the inner layer of the arachnoid to the pia mater. A similar 

 arrangement is found in certain situations at the base of the skull. 



The pia mater of the encephalon is a delicate, fibrous structure, exceedingly vascular, 

 seeming to present, indeed, only a skeleton net-work of fibres for the support of the ves 

 sels going to the nervous substance. This membrane covers the surface of the encephalon 

 immediately, follows the sulci and fissures, and is prolonged into the ventricles, where it 

 forms the choroid plexus and the velum interpositum. From its internal surface, small 

 vessels are given off which pass into the nervous substance. 



The pia mater of the encephalon is continuous with the corresponding membrane of 

 the cord ; but, in the spinal canal, it is thicker, stronger, more closely adherent to the 

 subjacent parts, and its blood-vessels are by no means so numerous. In this situation, 

 many of the fibres are arranged in longitudinal bands. This membrane lines the anterior 

 sulcus and a portion of the posterior sulcus of the cord. It is sometimes spoken of as the 

 neurilemma of the cord. At the foramina of exit of the cranial and the spinal nerves, 

 the fibrous structure of the pia mater becomes continuous with the nerve-sheaths. 



Between the anterior and posterior roots of the spinal nerves, on either side of the 

 cord, is a narrow, ligamentous band, the ligamentum denticulatum, which assists in hold- 

 ing the cord in place. This extends from the foramen magnum to the terminal filament 

 of the cord, and is attached, internally, to the pia mater, and externally, to the dura 

 mater. 



It is not necessary to enter into a detailed description of the arrangement of the blood- 

 vessels, nerves, and lymphatics of the membranes of the brain and spinal cord, or of the 

 vascular arrangement in the substance of the cerebro-spinal axis, as these points are 

 chiefly of anatomical interest. The circulation in these parts presents certain pecu- 

 liarities. In the first place, the encephalon being contained in an air-tight case of inva- 

 riable capacity, it has been a question whether or not the vessels be capable of contrac- 

 tion and dilatation, or whether the quantity of blood in the brain be subject to modifi- 

 cations in health or disease. These questions may certainly be answered in the affirmative. 

 In infancy and in the adult, when an opening has been made in the skull, the volume of 

 the encephalon is evidently increased during expiration and is diminished in inspiration. 

 Under normal conditions, in the adult, it is probable that the amount of blood is increased 



