108 



NERVOUS SYSTEM. 



Sinuses of the Base of the Skull. 



1. Ophthalmic veins. 2. Cavernous 

 sinus of one side, 

 the figure occupies 



Fig- 3L their blood to the heart by means of the 



lateral sinuses and internal jugulars; but of 

 the peculiarities of the circulation in the 

 encephalon, mention will be made in the 

 appropriate place. No lymphatic vessels 

 have been detected in the encephalon; yet, 

 that absorbents exist there is proved by the 

 dissection of apoplectic and paralytic indi- 

 viduals. In these cases, when blood has 

 been effused, the red particles are gradually 

 taken up, with a portion of the fibrinous 

 part of the blood, leaving a cavity called 

 an apoplectic cell, which is at the same time 

 the evidence of previous extravasation and 

 subsequent absorption. 



The whole of the nervous system is well 

 supplied "with bloodvessels. In the vesi- 

 cular neurine of the nervous centres, the 

 capillaries surround the ganglion cells or 

 globules; and in the tubular they pass 

 between the nerve-tubes, being connected 

 at intervals by transverse branches. 



When the skull of the new-born infant, 

 Supe- which, at the fontanelles, consists of mem- 



7. Internal jugular , , , , ', ,, . 



brane only or the head of any one who 

 has received an injury, that exposes the 

 brain is examined, two distinct move- 

 ments are perceptible. One, which is gene- 

 rally obscure, is synchronouswiththe pulsation 

 of the heart and arteries; the other, much 

 more apparent, is connected with respiration, 

 the organ seeming to sink at the time of in- 

 spiration, and to rise during expiration. This 

 phenomenon is not confined to the cerebrum, 

 but exists likewise in the cerebellum and spinal 

 marrow. The motion of the encephalon, 

 synchronous with that of the heart, admits 

 of easy explanation. It is owing to the 

 pulsation of the circle of arteries at the base of the brain elevating 

 the organ at each systole of the heart. The other movement is not 

 so readily intelligible. It has been attributed to the resistance, 

 experienced by the blood in its passage through the lungs during expi- 

 ration, owing to which an accumulation of blood takes place in the right 

 side of the heart; this extends to the veins and to the cerebral sinuses, 

 and an augmentation of bulk is thus occasioned. We shall see hereafter, 

 that one of the forces conceived to propel the blood along the vessels 

 is atmospheric pressure. According to that view, the sinking down of 

 the brain during inspiration is explicable: the blood is rapidly drawn 

 to the heart; the quantity in the veins is consequently diminished; and 

 sinking of the brain succeeds. 



or anterior occipital sinus. 



riorpetrosal sinus 



vein. s. Foramen magnum. 



Occi- 

 BrophiU ' 



Fig. 32. 



Capillary Net-work of Nervous 

 Centres. 



