PECULIARITIES OF CIRCULATION. 569 



inferred, however, that the absolute quantity of blood within the cranium is 

 not subject to variation; and that in the states of inflammation, congestion, or 

 other morbid affections, there is only a disturbance of the usual balance of 

 the arterial and venous circulation. The fact in all probability is rather, that, 

 the softness of the Cerebral tissue, and its varying functional activity, render 

 it peculiarly liable to undergo alterations in bulk ; and that the amount of the 

 cerebro-spinal fluid varies considerably at different times ( 476) ; so that the 

 quantity of blood may thus, even in the healthy condition, be continually 

 changing. Moreover, in disordered states of the circulation, the quantity of 

 blood in the vessels of the cranium may be for a time diminished by a sudden 

 extravasation, either of blood or serum, into the cerebral substance; and the 

 amount of interior pressure upon the walls of the vessels may also be con- 

 siderably altered, even when there is no difference in the quantity of fluid 

 contained in them.* 



748. The Erectile tissues constitute another curious modification of the 

 ordinary vascular apparatus. The chief of these are the Corpora Cavernosa 

 in the penis of the male, and in the clitoris of the female ; the collection of 

 similar tissues round the vagina, and in the nymphae, of the female ; and the 

 nipple in both sexes. In all these situations, erection may be produced by 

 local irritation ; or it may take place as a result of certain emotional conditions 

 of the mind ; the influence of which is probably transmitted through the 

 Sympathetic nerve, as it may be experienced even in cases of paraplegia. 

 The erectile tissue appears essentially to consist of a plexus of varicose Veins, 

 inclosed in a fibrous envelope. According to Gerber,t this plexus is traversed 

 by numerous contractile fibres, which are analogous to those that form the 

 dartos ; and to the contraction of these is probably to be attributed that ob- 

 struction to the return of blood by the Veins, which is the occasion of the 

 turgescence. The proximate cause of the erection of the Penis, has been 

 stated by some to be the action of the Ischio-Cavernosi muscles ; and by others 

 it has been attributed to the compression of the Vena dorsalis penis against 

 the Symphysis pubis. But it is obvious that nothing analogous to this can 

 apply to the other erectile organs, especially to the Nipple. In the Penis, 

 according to Miiller, there are two sets of arteries ; of which one, destined for 

 the nutrition of the tissues, communicates with the veins in the usual way, 

 through a capillary net-work ; whilst the others pass off as large branches, and 

 penetrate the cavernous substance in a helicine manner, communicating ab- 

 ruptly with the venous cells. It would seem not improbable, that these last 

 are not ordinarily pervious to blood ; but that the same change in the contrac- 

 tile fibres, which impedes the return of the blood by the veins, may also 

 permit it to enter more freely from the helicine arteries. This double com- 

 munication, however, is denied by Valentin, who gives a different explanation 

 of the appearances described by Miiller. The arteries are protected in such 

 a manner, that, even when the veins are most compressed, and the erection 

 most complete, they are still quite pervious. 



The results of the more recent experiments of Dr. G. Burrows (Med. Gaz., April and 

 May, 1843) fully confirm the views stated above. 

 | General Anatomy, p. 298. 



48* 



