Many of these arteries enlarge towards their end; this enlargement is gradual, and is greatest at some little distance from the extremity, so that the end is somewhat conical, terminating immediately in a rounded point without giving off any branches. The diameter of these arte rial twigs, in their middle, is from one-fifth to one-sixth of a millimetre: those which branch off from the trunk of the arteria profunda penis are no larger than those which arise from its finest twigs. It is by no means unusual to observe the finest twigs of the arteria proftinda giving off branches of this kind which seem much thicker than the twig from which they arose. The annexed figure (fig. 98) (from Miiller's Archly.) repre sents a portion of the arteria profunda penis of man, with its arteries helicinm somewhat mag nified.
These remarkable arte ries have a great resem blance to the tendrils of the vine, only that they are so much shorter in proportion to their thick ness, whence they have received the name arteries lielicinm. Their termi nations may also be com pared to a crosier. By a more minute examination of these vessels either with the lens or with the microscope, it will be seen that, although they at all times project into the venous cavities of the corpora cavernosa, yet they are not entirely naked, hut are covered with a delicate mem brane, which under the microscope appears granular (fig. 99).
After a more forcible in jection this envelope is no longer visible. When the arteries form a bundle, the whole is covered by a slight gauzelike membrane.
With respect to this in vesting membrane, Profes sor Miiller appears to con sider it as performing an important part in producing the phenomena of erection.
These tendril-like arteries have neither on their surface nor their extremities any openings discoverable with the aid of the microscope ; and when the blood, as it is probable, escapes from them in large masses into the cells of the corpora cavernosa during erection, it inust either traverse invisible openings, or pass through small openings which become en larged by the dilatation of these arteries. If the great number of the tendril-like branches of the arteria profunda be compared with the very fine nutritious twigs of the same vessel, it is evident that when the former are filled they must take up the greater part of the blood of the arteria profunda; the diameter of the profunda therefore not only includes its nu tritious twigs, but also the tendril-like branches, which derive their blood from it, yet pro bably allow none to pass except during erec tion; therefore the blood in the unerected state only traverses the nutritive branches and ar rives at the commencement of the venous cells in smaller quantities, while during erection it probably passes in considerable quantity into the cells through these tendril-like vessels.
l'rofessor Muller, after pointing out the dif ference between the tendril-shaped vessels and the looped vessels discovered by Weber in the villi of the placenta, observes : our vessels are. simple ; they bend themselves at the end, but do not return to their trunk as a loop, being simply blood-containing processes of the ar teries which project freely into the cellular cavities of the veins of the corpora cavernosa. These vessels are most numerous in the pos terior part of the corpora cavernosa ; they occur but seldom in the middle and anterior parts : they are also present in the corpus spongiosum urethrse, especially in the bulb ; here also they become less frequent anteriorly, and as yet they have not been perceived in the glans. They are much more difficult of detection in the corpus spongiosum urethrae than in the corpora cavemosa, where they are very easily exhibited, especially in the human penis. In no other animal have they been found so dis tinct, or so uniform in their existence as in man. The greater development of these arteries, adds Professor Miller, in the posterior parts of the organ corresponds with the fact of erection being always earlier evident there, as if the blood distributed itself from thence into the venous cells.
During erection blood is accumulated in large quantity in the erectile tissue, but the cause and mechanism of this accumulation are hut imperfectly known. Ilehenstreit ascribes it to a living power, named turgor vitalis, which exists in different degrees in almost all the textures of the animal body, but most dis tinctly in the erectile tissue. It still remains, however, to be proved how far erection de pends on mechanical pressure affecting the veins which convey blood from this structure, and consequent retardation of the venous circu lation ; and how far it may depend upon an increased flow of blood to its arteries accompa nied, or perhaps more correctly, occasioned by an increase of sensibility,* or whether it may not depend upon the influence of both these causes combined.