280 



MANUAL OF HISTOLOGY. 



last months, the fibres are quite similar to what are found in the adult 

 (fig. 270, a, c), at times, however, still preserving the cellular character 

 (5). Not very unfrequently we encounter also lens fibres with double or 

 even triple nucleus (d). The further production of these tubes probably 

 takes place by a process of segmentation of the immature cells in the 

 zone situated at the border of the epithelium of the capsule ( 159), the 

 new elements being laid down over the older ones. That the process and 

 the growth of the lens extend far beyond the period of intra-uterine life 

 is almost a matter of certainty. 



At an early period the capsule of the lens is enclosed in a vascular 

 membrane, which forms a part of the well known system of envelopes of 

 the organ under the name of the membrana capsulo-pupillaris. 



After birth the number of the fibres of the lens is multiplied with the 

 growth of the body, but their diameter is not increased. 



They take their rise from the epithelial cells of the capsule, and, in 

 keeping with the character of epithelial structures, can be regenerated 

 provided the capsule and layer of cells be preserved. It is not difficult 

 to conceive that a lens formed after the capsule has once been opened never 

 attains the same regularity of form as the first structure, seeing its figure 

 is quite dependent on that of the capsule. The amount and nature of the 

 interchange of matter going on in this organ is not yet known. The 

 former is probably not entirely insignificant. 



14. Muscle Tissue. 



162. 

 The muscles, springing from the middle embryonic plate, are made up 



d 



Fig. 271. Striped muscle-fibres. 



Fig. 272. Elements of 

 smooth muscle, from 

 the rabbit. 



of a soft reddish fibrillated tissue, which is remarkable for the property it 

 possesses of contracting when its motor nerves are excited. This pecu- 

 liarity is characterised by the term irritability. As we are taught by 



