726 



PROTECTIVE AND NUTRITIVE APPARATUS OF THE BRAIN. 



the head leans towards the anode, the eyes turn in that direction and often exhibit nystagmus. 

 The eleetricalcurrent probably stimulates the nerves of the ampullar, as we know that affections 

 of^^b^escaule vertigo ( 350). The cerebellum has no relation to the sexua activities 

 aIwrm^utai^riVGall g The contractions of the uterus observed by Valentin, Budge, and 

 Snieeelberz, after stimulation of the cerebellum, are as yet unexplained ...... . , n 



Pathological. -Lesions of one hemisphere may give rise to no symptoms ; but if the middle 

 lobe is involved, there is inco-ordination of movement, especially a tendency to fall, unsteady 

 it and pron. >Jnced vertigo. Irritative lesions of the middle peduncle cause complete gyrating 

 movements of the body around its axis, together with rotation of the eyes and head (Aothnagel). 

 381 PROTECTIVE APPARATUS OF THE BRAIN. The Membranes. The dura mater 

 cerebralisis intimately united to the periosteum of the cavity of the skull while the spinal 

 dura mater forms around the spinal cord a freely suspended long sac, fixed only on its anterior 

 surface It is a fibrous membrane, consisting of firm bundles of connective-tissue intermixed 



with numerous elastic fibres, and 

 $ a provided with flattened connective- 



tissue corpuscles and Waldeyer's 

 plasma cells. The smooth, inner 

 surface is covered with a layer of 

 endothelium. It is but slightly 

 supplied with blood-vessels, although 

 they are more numerous in the outer 

 layers; the lymphatics are numerous, 

 while nerves whose terminations are 

 unknown give to the dura its ex- 

 quisite sensibility to painful opera- 

 tions on it. Pacinian corpuscles 

 have been found in the dura over 

 the temporal bone. The lymphatic 

 subdural space {Key and Rctzius) 

 lies between the dura and the arach- 

 noid, and between the pia and arach- 

 v . rnQ noid is the subarachnoid space (fig. 



** oWm 509). These two spaces do not 



Vertical section of the cortex cerebri and its membranes ; communicate directly. The delicate 

 x 24. co, cortex cerebri; p, intima pise dipping into the aracnil0 id, thin and partially per- 

 sulci ; a, arachnoid, connected with p by means of the fo^t^ poor i u blood-vesseis and 

 loose subarachnoid trabecule in the subarachnoid space, w ithout nerves, is covered on both 

 na ; v, v, blood-vessels ; d, dura ; sd, subdural space. surfaces with squamous endothelium. 

 < Milv on the spinal cord is it separated from the pia, so that between the two lies the lymphatic 

 subarachnoid space ; over the brain, the two membranes are for the most part united together, 

 except the parts bridging over the sulci between adjacent convolutions. The arachnoid passes 

 from convolution to convolution without dipping into the sulci, while the pia dips into each 

 sulcus (fig. 509, a). The ventricles of the brain communicate freely with the lymphatic 

 subarachnoid space, but not with the subdural space. The pia consists of delicate bundles of 

 connective-tissue without any admixture of elastic fibres; it is richly supplied with blood- 

 vessels and lymphatics, and carries nerves which accompany the blood-vessels into the substance 

 of the brain. The lymphatics open into the subarachnoid space ( 196). 



[Subarachnoid Fluid, or cerebro-spinal fluid, lies in the subarachnoid space, which is 

 traversed by trabecular of connective-tissue. Within the brain are a series of cavities called 

 ventricles, which communicate one with another in a definite way. The fourth ventricle is 

 lined by a layer of columnar epithelium, and covered in dorsally by a membrane and continua- 

 tion of the pia mater, from the middle of which there hangs into the roof of the fourth ventricle 

 two vascular processes composed of capillaries the choroid plexuses of the fourth ventricle, 

 which are comparable to the larger plexuses of the lateral ventricles. In this membrane is the 

 foramen of Magendie and two other smaller foramina, whereby the fluid in the subarachnoid 

 space communicates with that in the fourth ventricle ; but the lymphatics of the nerve-sheaths 

 can be injected from the subarachnoid space, so that there is direct continuity of the fluid in 

 the ventricles of the brain with that in the subarachnoid space, perivascular spaces of the 

 cerebral substance, and the perineural lymphatics of nerves. The average quantity is about 2 

 ounces, and if it be suddenly withdrawn, epilepsy or convulsions may be produced ; or, if it be 

 rapidly increased in amount, coma may be produced. The middle and posterior parts of the 

 brain and the medulla oblongata do not rest directly on bone, but are separated by a distinct 

 interval from their osseous case, an interval occupied by the cerebro-spinal fluid and traversed 

 by trabecular, so that, as Hilton expresses it, this fluid forms a perfect water-bed for those parts, 

 being sustained by the venous circulation and the elasticity of the dura. It has important 

 mechanical functions, protecting delicate parts of the brain from injury; by distributing 



