296 INNERVATION. [CHAP. X. 



arteries and veins can also be admirably studied in the pia mater 

 of the brain. 



The venous blood is collected into small veins, which are formed 

 in the pia mater at various parts of the surface, and in the interior 

 of the brain. The superficial veins open by short trunks into veins 

 of the dura mater, or into the neighbouring sinuses ; the superior 

 longitudinal, the lateral, and the strait sinuses receiving the greatest 

 number. Those from the interior form two trunks, vena magnet 

 Galenij which pass out from the ventricles between the layers of the 

 velum interpositum. The cerebral veins are devoid of valves. 



We remark here, that the venous blood of the brain is returned 

 to the centre of the circulation through the same channels as that 

 of the dura mater, of the cranial bones, and of the eyeball : the 

 deep jugular veins are the outlets by which the venous blood of 

 the cranium is discharged. An obstacle, therefore, in both or 

 either of these trunks must affect the entire venous system of 

 the brain, or at least that of the corresponding hemisphere. A 

 ligature tied tightly round the neck impedes the circulation, and 

 may cause congestion of the brain. The bodies of criminals who 

 have died by hanging exhibit great venous congestion, both of the 

 walls and the contents of the cranium, in consequence of the strong 

 compression to which the veins have been submitted. 



We have seen, that, when the blood of one carotid artery is cut 

 off, the parts usually supplied by it are apt to become exsangueous 

 and softened ; and this is more especially the case if the vertebral 

 be also impacted, or the circulation in it impeded. And it has been 

 remarked, that these effects will follow the application of a ligature 

 to either common carotid artery. 



Notwithstanding these facts, a doctrine has received very general 

 assent, and the support of men of high reputation, which affirms 

 that the absolute quantity of blood in the brain cannot vary, be- 

 cause that organ is incompressible, and is enclosed in a spheroidal 

 case of bone, by which it is completely exempted from the pressure 

 of the atmosphere. 



The cranium, however, although spheroidal, is not a perfectly 

 solid case, but is perforated by very numerous foramina, both ex- 

 ternal and internal, by which large venous canals in the diploe 

 of the bones communicate with the circulation of the integuments 



O 



of the head as well as with that of the brain; so that the one 

 cannot be materially affected without the other suffering likewise. 

 And as the circulation in the integuments is not removed from 

 atmospheric pressure, neither can that which is so closely connected 



