728 THE BRAIN. 



processes to the psychical action which we call attention. On the other 

 hand, the period is distinctly prolonged if the observer be distracted by con- 

 comitant sensations. For example, the period for discriminating between 

 two visual sensations is prolonged if powerful auditory sensations be excited 

 at the same time. 



The same method of measurement may be used in other ways and under 

 other circumstances with reference to psychical processes. It must be re- 

 membered, however, that all such observations are open to many fallacies 

 and need particular caution. It not unfrequently happens that false results 

 are obtained ; for instance, the subject, expecting the stimulus to be brought 

 to bear upon him and straining his attention, makes the signal before the 

 stimulus actually comes off. And the interpretation of the results obtained 

 are in many cases very difficult ; but it would be out of place to dwell upon 

 these matters any further here. 



THE LYMPHATIC ARRANGEMENTS OF THE BRAIN AND SPINAL CORD. 



605. The membranes of the brain and spinal cord. The cerebro-spinal 

 canal is lined by a tough lamellated membrane, composed of connective 

 tissue with a small amount of elastic network, called the dura mater, which, 

 somewhat closely adherent to the walls of the cranial cavity, is separated 

 from those of the vertebral canal by a considerable space, containing blood- 

 vessels, especially large venous sinuses, and some fat. It may be considered 

 as a development of the periosteum lining the cerebro-spinal cavity. It 

 sends tubular sheaths lor some distance along the several cranial and spinal 

 nerves ; and forms between the cerebral hemispheres, in the longitudinal 

 fissure, a conspicuous sickle-shaped vertical fold, the falx cerebri, as well as 

 a smaller horizontal or oblique fold between the cerebellum and cerebrum 

 known as the tentorium. 



The vascular pia mater is closely attached to the surface of the brain and 

 spinal cord, dipping down, as we have seen, into the ventral or anterior fissure 

 of the cord as well as into the fissures of the brain. Sheath-like investments 

 of pia mater are continued along the several nerves as they leave the cerebro- 

 spinal cavity ; and in the vertebral canal an imperfect partition half-way 

 between the dorsal and ventral surfaces of the cord is furnished by a mem- 

 brane of connective tissue which, continuous along its whole length with the 

 pia mater, is attached to and fused with the dura mater at intervals only, 

 namely, between the successive nerve-roots. Since its outer edge has thus a 

 toothed appearance, this membrane is called the ligamentum denticulalum. 

 Between the pia mater next to the brain and cord and the dura mater next 

 to the bony walls is a cavity, which is divided into two by a thin membrane, 

 the arachnoid, composed of interwoven bundles of connective tissue. The 

 space between the arachnoid and the dura rnater is called the subdural space, 

 and the space between the arachnoid and the pia mater is called the sub- 

 arachnoid space. When the brain is exposed by removing the roof of the 

 skull and slitting open the dura mater, the subdural space is laid bare, and 

 the arachnoid is seen stretching over the pia mater ; in the vertebral canal 

 the arachnoid lies close to the dura mater, so that usually, when the dura 

 mater is slit open and turned back, the arachnoid is carried with it and the 

 cavity exposed is that of the subarachnoid space. The arachnoid, like the 

 dura mater and the pia mater, is continued for some distance over the nerves 

 as they leave the cerebro-spinal cavity; so that each nerve at its exit is sur- 

 rounded by a tubular prolongation of the subdural space, and within this a 

 similar tubular prolongation of the subarachnoid space. 



The subdural space is broken up to a slight extent only by bridles carry- 



