PECULIARITIES OF CIRCULATION. 501 



filled with the same total amount of contents, the flow of blood through its ves- 

 sels is attended with some peculiarities. The pressure of the atmosphere is 

 here exerted, rather to keep the blood in the head, than to force it out ; and it 

 might accordingly be inferred that, whilst the quantity of cerebral matter re- 

 mains the same, the amount of blood in the cranial vessels must also be invari- 

 able. This inference appeared to derive support from the experiments of Dr. 

 Kellie. 1 On bleeding animals to death, he found that, whilst the remainder of 

 the body was completely exsanguine, the usual quantity of blood remained in 

 the arteries and veins of the cranium ; but that, if an opening was made in the 

 skull, these vessels were then as completely emptied as the. rest. It is not to be 

 hence 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; so that the quan- 

 tity 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 considerably altered, even 

 when there is no difference in the quantity of fluid contained in them. 3 



534. The Erectile tissues constitute another curious modification of the ordi- 

 nary 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 nymphse, 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 veins with varicose enlargements, inclosed 

 in a fibrous envelop with trabecular partitions. This envelop, according to 

 the recent researches of Prof. Kb'lliker, contains a large amount of non-striated 

 muscular fibre ; the contraction of which is doubtless in some way concerned in 

 the result. In the penis, as first pointed out by Prof. Miiller, there are two sets 

 of arteries ; those of one set, destined for the nutrition of the tissues, commu- 

 nicating with the veins in the usual way, through a capillary network ; whilst' 

 the others, termed by him the " helicine arteries," are short tendril-like branches, 

 which project into the veins (covered, however, by their lining membrane), 

 sometimes singly, and sometimes in tufts, ending abruptly by dilated extremities. 

 It was maintained by Miiller that the dilated ends of these helicine arteries 

 open into the venous cavities ; but no distinct apertures have been seen in them ; 

 and it seems more probable that (as Miiller himself admits) they are merely 

 arterial diverticula, the distension of which concurs with that of the venous 

 areol93, in the act of erection. 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 although these muscles probably afford 

 assistance in completing and strengthening the erection, it is obvious that no 

 analogous power can be exerted in other erectile organs, the nipple, for example. 



1 "Edinburgh Medico-Chirurgical Transactions," vol. i. 



2 The results of the more recent experiments of Dr. G. Burrows ("Medical Gazette," 

 April and May, 1843) fully confirm the views stated above. 



