PROTECTIVE AND NUTRITIVE APPARATUS OF THE BRAIN. 8ll 



as well as the cerebrospinal fluid, and, on the other hand, the system of blood- 

 vessels. If the volume of the latter is increased in consequence of the presence 

 of an increased amount of blood in the skull, the brain becomes poorer in fluid, 

 like an expressed sponge. Conversely, in case of excessive production of the 

 fluids mentioned the Wood must escape from the vascular system. That the 

 latter, however, is possible must be concluded from the circumstance that the 

 formation of the fluid is not, under all circumstances, dependent, as a simple 

 transuding filtrate, solely upon the blood-pressure, but that it may take place also 

 independently of the latter as a result of the secretory activity of the vessels. 



The brain and the fluid surrounding it are constantly under a certain mean 

 pressure, which is influenced by the atmospheric pressure, so that the pressure 

 within the skull is altered in correspondence with fluctuations in the atmospheric 

 pressure. According to Grashey there prevails in the skull of an adult a negative 

 pressure of 13 cm. of water; at the foramen magnum it is zero. In the dural 

 sac of the spinal cord there is a positive pressure, below (in the erect posture) 

 greater than above, but on the average +60 cm. of water. The investigations 

 of Naunyn and Schreiber upon pathological brain -pressure, or cerebrospinal 

 pressure, have shown that this pressure must reach a level somewhat below the 

 arterial pressure in the carotid artery before the distinctive symptoms of cerebral 

 pressure appear. These consist of headache of paroxysmal occurrence, with 

 marked vertigo to the point of unconsciousness, vomiting, slowing of the pulse, 

 slow and shallow respiration, convulsions, injection of the conjunctiva, and in- 

 crease of the pressure of the cerebrospinal fluid. The cause of these symptoms 

 resides in anemia of the brain, so that bloodletting is to be avoided. In conse- 

 quence of excessive tension of the cerebrospinal fluid the brain is expressed like a 

 sponge. The blood escapes from it, and naturally from the capillaries most 

 readily, as these can be most readily expressed on account of their lower internal 

 pressure. Acute cerebral anemia is thus induced. If the degree of pressure 

 attains only a moderate level the symptoms described may remain latent. Never- 

 theless, nutritive disorders develop in the brain, with consecutive phenomena, 

 such as persistent slight headache, a feeling of vertigo, muscular weakness, visual 

 disturbances (in consequence of neuroretinitis with papillitis). The symptoms 

 may be relieved by elevation of the blood-pressure, while reduction of the pressure 

 causes more marked symptoms of cerebral pressure. 



At a pressure of from 70 to 80 mm. pain appears in dogs only in consequence 

 of mechanical irritation of the dura; at a higher pressure, loss of consciousness; 

 at a pressure of 100 mm., convulsions similar to those attending sudden occlusion 

 of the arteries. A pressure of from 100 to 120 mm. gives rise to slowing of the 

 pulse in consequence of central irritation of the vagus, while the respiratory 

 frequency exhibits a transitory increase and later a reduction. Marked com- 

 pression of long standing terminates fatally sooner or later. The blood-pressure 

 is first increased in consequence of reflex stimulation of the vasomotor center, 

 as a result of irritation of the sensory nerves by pressure. Then the blood-pressure 

 falls, with marked slowing of the pulse. In addition, variations in blood-pressure 

 of irregular occurrence are indicative of direct central irritation of the vasomotor 

 center by pressure. 



At the level of the cauda equina the pressure of the spinal fluid in the arach- 

 noid sac is only between 7.5 and 12 mm. of mercury in the dog. After evacuation 

 of the cerebrospinal fluid restoration takes place rapidly. Artificial increase is 

 soon neutralized, the excess of fluid passing into the lymphatics and the veins. 



COMPARATIVE. HISTORICAL. 



Nerves are wanting in the protozoa. Among the celenterates the first in- 

 dications of a nervous system are present in the neuromuscular cells of the hydroids 

 and the medusze. In the latter a closed nervous chain passes along the mar- 

 gin [of the umbrella and, corresponding to the marginal bodies, exhibits cell- 

 like thickenings from which filaments pass to the sense-organs. In worms a 

 ring is often attached to the head and it surrounds the pharynx in those provided 

 with intestines as a single or a double ring. From this there pass into the elongated 

 body longitudinal trunks, frequently two, which are provided with ganglia corre- 

 sponding to the body-segments, and here they anastomose. In the leech only 

 one longitudinal trunk provided with ganglia, the so-called abdominal medulla, is 

 present. In echinoderms the mouth is surrounded by a large nervous ring, from 



