HISTOLOGY OF THE FROG 99 



The various organs and tissues which we have enumerated 

 are supported by and attached to the skeleton, and are bound 

 together and pervaded by connective tissue. The skeleton is 

 composed of bone and cartilage, which are classified as varieties 

 of connective tissue ; but we must regard the white membranous 

 or fibrous material, which forms the sheaths of muscles and 

 nerve-trunks, ties down the skin to the underlying tissues, and 

 enters into the composition of tendons and aponeuroses, as 

 constituting connective tissue proper, bone and cartilage 

 standing somewhat apart from these. 



Connective tissue may be studied to more advantage in 

 mammals than in the frog. On cutting through the skin of a 

 rabbit and stripping it from the body it is seen to be connected 

 to the subjacent parts by a soft, white, tenacious, fleecy 

 material known as areolar tissue (fig. 19). If a portion of 

 this be carefully spread out as a film on a slide, and examined 

 under the microscope, it is seen to be composed of a soft homo- 

 geneous ground substance or matrix in which are imbedded 

 cells and bundles of extremely fine transparent white fibres, 

 which traverse the matrix in all directions. These fibres do 

 not branch, nor do those of one bundle cross over to and 

 unite with those of another bundle, but keep a direction 

 parallel with the remaining fibres of the bundle. They swell 

 up in acetic acid, and are converted into gelatin on boiling. 

 Each bundle usually pursues a wavy course, its component 

 fibres maintaining their parallelism throughout the undulations. 

 In addition, larger fibres may be distinguished which run 

 nearly straight, or are thrown into large bold curves unlike 

 the minute undulations of the white fibres. They differ also 

 from the latter in the fact that they branch and anastomose 

 with one another to form an open network, and also, when 

 broken across, their ends tend to curl up. These are the 

 yellow elastic fibres ; they are not affected by acetic acid, 

 and do not yield gelatin on boiling, being composed of a 

 different chemical substance called elastin. 



The ground substance of areolar connective tissue stains, 

 like intercellular substances in general, with silver nitrate, and 

 it then exhibits a number of irregular cavities pervading a 

 homogeneous basis. By staining connective tissue with other 

 dyes it may be shown that these spaces are occupied by 

 branching cells, the so-called connective tissue corpuscles. 



