PROTECTIVE AND NUTRITIVE APPARATUS OF THE BRAIN. 809 



well as atony, asthenia, and ataxia. Irritative disease of the middle cerebellar 

 peduncle causes complete rotation of the body about its axis, with rotation of 

 the eyes and the head in the same direction. 



If an electrical current be passed through the head of a man, the electrodes 

 being placed in the mastoid fossae behind each ear, and in such a manner that the 

 positive pole is applied upon the right and the negative pole upon the left, a 

 marked feeling of vertigo occurs on closure, and the head and body fall toward 

 the positive pole, while the objects of the outer world appear to move toward 

 the left. If during the passage of the current the eyes are closed, the apparent 

 movement is transferred to the individual himself, who then has a feeling of 

 rotation toward the left. At the moment when the head falls toward the anode, 

 the eyes also are rotated in the same direction and frequently exhibit nystagmus. 

 The electrical current under such circumstances probably exerts an irritative 

 effect upon the nerves of the ampullae, disorders of which cause vertigo. 



PROTECTIVE AND NUTRITIVE APPARATUS OF THE BRAIN. 



The cerebral dura mater is intimately united with the periosteum of the cranial 

 cavity. The spinal dura mater forms about the spinal cord a freely suspended 

 long sac attached only on its anterior aspect. The dura mater is a fibrous mem- 

 brane consisting of firm bands of connective tissue, interwoven with a large num- 

 ber of elastic fibers and provided with flat connective-tissue cells and Waldeyer's 

 glasma-cells. The smooth inner surface is lined by squamous endothelium. 

 lood-vessels are present only in moderate number, though in somewhat greater 

 abundance in the outer layers, while lymphatics are numerous. Nerves with 

 unknown terminations (Pacinian bodies have been found on the petrous bone) 

 endow the dura with great sensitiveness to painful impressions (also in the dog, 

 but not in the rabbit). 



Between the dura and the arachnoid is situated the lymphatic subdural space. 

 The pia mater and the arachnoid united to it by means of a reticular network 

 really form a common membrane, which cannot be separated. Between the 

 two layers, as if enclosed in dropsical connective tissue, cerebrospinal lymph 

 is present in a space, the subarachnoid space, which is lined by endothelium. The 

 external limiting layer of this stratum, correctly designated also arachnoid in 

 the strict sense, is thin, poor in vessels, without nerves and lined on both surfaces 

 by squamous endothelium. It is, however, separated from the pia only over the 

 spinal cord, so that between the two lies the lymphatic subarachnoid space. 

 Over the brain the two are in large measure united, except where they form bridges 

 over the sulci. Over these the arachnoid merely passes, while the pia penetrates 

 into the depth. The cerebral ventricles communicate freely with the lymphatic 

 subarachnoid space, but not with the subdural space. The subdural and sub- 

 arachnoid spaces do not communicate with each other. The pia, made up of deli- 

 cate bundles of connective tissue, without elastic fibers, exceedingly rich' in 

 blood-vessels and lymphatics, conveys nerves in association with the vessels 

 into the structure of the central organs. 



The lymphatics of the brain, apart from those accompanying the vessels, consist 

 of spaces surrounding the ganglia and of the glia-cells of the cortex with their pro- 

 cesses. They all empty finally into the subarachnoid space. The cerebrospinal 

 fluid is described on p. 367. The Pacchionian granulations are connective-tissue 

 villi that serve for the flow of lymph from the subdural and subarachnoid spaces 

 into the sinuses of the dura mater, particularly the superior longitudinal sinus, 

 into which they project. The subarachnoid space communicates also with the 

 spongy cavities of the cranial bones and with the veins of the surface of the skull 

 and of the face. The subdural space communicates, further, with lymphatic 

 spaces of the dura, and the latter communicate directly with the veins of the 

 dura. The two lymphatic intermeningeal spaces communicate also with the 

 lymphatics of the nasal mucous membrane. The space external to the spinal 

 dura (epidural space) may also be considered as a lymphatic space. From 

 it the pleural and peritoneal cavities may be readily filled. It does not, however, 

 communicate with the cranial cavity. The venous plexuses, which perhaps 

 secrete the cerebrospinal fluid, consist of convolutions of vessels surrounded by 

 undeveloped connective tissue. The telae choroideas in the newborn are still 

 provided with ciliated epithelium. 



The pulsations of the large blood-vessels at the base of the brain impart 

 pulsatory movements to the latter. As a result of the physical conditions present 



