388 THE TISSUES. 



of many veins. In the smallest arteries they are elongated, or even 

 round cells; which is to be regarded as a less developed form. 



7. In the skin, this tissue appears in the form of minute fasciculi 

 upon the hair-sacs, and which are hence called arrectores pili (p. 

 267) ; and in many of the sudoriparous and sebaceous follicles. 

 Similar arrectores have also been found by Mr. Lister in the scalp ; 

 these little muscles being about ^^ of an inch in diameter. Their 

 existence in the dartos, the areola and the mammilla, has already 

 been mentioned; and Kolliker asserts that they exist in all situa- 

 tions where hairs occur. 



8. In the eye, smooth fibre-cells form both the sphincter and the 

 dilator (the circular and the radiating fibres) of the pupil, and the 

 tensor choroidece (ciliary mjuscle). 



[9. The spleen in many animals has this tissue in its outer coat, 

 and in its trabeculse, mixed with areolar tissue ; but it is not found 

 in the human spleen.] 



10. Finally, this tissue forms an incomplete coat in Wharton's 

 duct, and in the ductus communis choledochus; while the gall- 

 bladder is completely lined by a layer of it. 



Peculiarities. The fibre-cells of the uterus demand a special 

 notice. 



Muscular Fibre- Cells of the Uterus. 



The fibre-cells of the uterus, while in its normal state, are quite 

 short; being only 5^3 to ^fa of an inch in length, and g^ UTJ of 

 an inch wide. As it enlarges after impregnation, and finally aug- 

 ments to twenty -four times its original size (Meckel), all its histolo- 

 gical elements undergo an increased development. But only the 

 changes in its fibre-cells will be considered here. 



Kolliker has ascertained that the walls of the uterus increase in 

 thickness up to the fifth month of gestation, and then gradually 

 become thinner, while its cavity increases up to the full term; and 

 that so far as the muscular structure is concerned, there is both an 

 enlargement of the original fibres and a production of new ones. 

 At the end of 5J to 6 months after impregnation, the fibres have 

 become ^J^ to T ^ of an inch long, g^Vu to ^Vir of an inch wide, 

 and gfljg to 4 ^gg of an inch thick; instead of the dimensions men- 

 tioned above. Consequently, their length is increased from seven 

 to eleven times ; and their width from two to jive times. Acetic acid 

 brings out a distinct cell-wall inclosing these large fibres. 



