388 ON RESPIRATION. 



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 sympa- 

 thetic nerve, as it may be experienced even in cases of paraplegia. The erectile 

 tissue appears essentially to consist of a plexus of varicose veins, enclosed in 

 a fibrous envelop. According to Gerber,* 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 obstruction to the return 

 of blood by the veins, which is the occasion of the turgescence. The proxi- 

 mate 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, communi- 

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

 others pass off as large branches and penetrate the cavernous substance in a 

 helicine manner, communicating abruptly 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 contractile 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 communication, 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. 



CHAPTER X. 



ON RESPIRATION. 



I. Nature of the Function; and Provisions for its Performance. 



520. IT is obvious that the Nutritive fluid, in its circulation through the 

 capillaries of the system, must undergo great alterations, both in its physical 

 constitution, and in its vital properties. It gives up to the tissues with which 

 it is brought into contact, some of its most important elements ; and, at the 

 same time, it is made the vehicle of the removal, from these tissues, of ingre- 

 dients which are no longer in the state of combination, that fits them for their 

 offices in the Animal Economy. To separate these ingredients from the 

 general current of the circulation, and to carry them out of the system, is the 

 great object of the Excretory organs; and it is very evident that the importance 

 of the respective functions of these will vary with the amount of the ingredient 

 which they have to separate, and with the deleterious influence which its 



* Op. cit., p. 298. 



