CHAP. VII.] DISTRIBUTION OF MUSCLE. 161 



corroborate it : " When the nervi vagi are pinched, one sudden 

 action of the fibres of the oesophagus ensues, and, presently after, 

 a second, of a slower character, may be observed to take place."* 

 The characters of these movements would appear to indicate the 

 existence of both varieties of fibre. 



The cross stripes on the fibres of the heart are not usually so 

 regular, or distinct, as in those of the voluntary muscles. They 

 are often interrupted, or even not visible at all. In some of the 

 lower animals their sarcous elements never form transverse stripes. 

 These fibres are usually smaller than the average diameter of those 

 of the voluntary muscles of the same subject by two-thirds, as stated 

 by Mr. Skey ; and in most parts of the parieties of this viscus they 

 are not aggregated in parallel sets, but twine and change their re- 

 lative position. This may be seen in a well-boiled heart. Striped 

 fibres have been found in the iris, in the small muscles of the ear, 

 and in those muscular fasciculi that surround the urethra imme- 

 diately in front of the prostate; They are also found in the 

 sphincters of the anus and vagina. 



The unstriped fibre is met with in the alimentary canal, and con- 

 stitutes the double layer investing that tube. It also forms the 

 muscular coat of the bladder, and that of the uterus. The dartos 

 owes its contractility to the presence of fibres of this variety ; which, 

 in consequence of their admixture with a great abundance of areolar 

 tissue, have been often overlooked. But they may be detected by 

 the addition of acetic acid, which, by bringing into view the pecu- 

 liar corpuscles they contain, distinguishes them from both the white 

 and yellow fibrous elements of the areolar tissue. A very distinct 

 peristaltic contraction may be often discerned in the dartos, ex- 

 tending across the raphe of the scrotum, and too similar to the con- 

 traction of unstriped muscles to be mistaken. 



The fibres which have been described as peculiar to the dartos 

 seem to be nothing more than a certain modification of the areolar 

 tissue in that region. The erection of the penis may be, in part, 

 owing to the compression exerted on the superficial veins of the 

 organ by a continuation of a structure analogous to the dartos, 

 which is continued over the base of the penis under the skin. The 

 erection of the nipple also occurs, on any mechanical irritation, with 

 a motion so very like muscular contraction, that a layer of these 

 might perhaps be found under the skin of that region. And it 

 may be matter of question how far the general contractility of 



* Physiology, 3d ed. p. 41. 

 VOL. I. M 



