356 AN INTRODUCTION TO ZOOLOGY 



be an external surface, for it can line an internal cavity. Epithelia 

 may be divided in the following way. 



Simple epithelia consist of but one layer of cells, and are sub- 

 divided according to the shape of the cells into : Squamous, con- 

 sisting of flat cells ; Cubical, in which the cells are approximately 

 cubical in shape ; and Columnar, with cells much longer than they 

 are wide. 



Compound epithelia have their cells arranged in a number of 

 superimposed layers, and are subdivided into : Stratified, in which 

 the cells are arranged in a'series of layers, the innermost cubical or 

 polygonal, and the outermost quite flat. This is the most common 

 and typical, and Transitional, consisting of two or three layers, all 

 of more or less polygonal cells, and not becoming flattened on the 

 outside. This latter variety is found in the bladder and the cornea 

 of the eye. 



It is the epithelia, particularly the simple ones, that provide 

 most of the secretory tissues of the glands, and, consequently, we 

 find that almost any type of epithelium may be glandular in nature. 

 They also form the essential part of the sense organs, where they 

 are termed neuro-epithelia. A further modification is to be found 

 in the pharyngeal region of Rana, and in the trachea of Lepus, 

 where the cells are provided with an external covering of cilia 

 constituting a ciliated epithelium. 



As a rule, we find that the glandular epithelium does not remain 

 upon the surface, but becomes invaginated to form a gland. This 

 may be a simple ingrowth, and so form a tubular gland, such as we 

 find in the mucosa of the stomach, or, if its deeper end or fundus is 

 much enlarged, it produces a flask or saccular gland, similar to those 

 in the skin of Rana. On the other hand, the invagination may 

 become complexly branched and, if its branches remain tubular, 

 form a compound tubular gland, as in the kidney, or, if the ends of 

 the branches dilate, a compound saccular or racemose gland, as in 

 the pancreas and salivary glands. 



The connective tissues, as we have seen, are characterised by 

 possessing a fluid matrix in which their cells float, and through 

 which pass typically white and yellow fibres. Any one of these 

 elements may be modified, developed to a marked degree, or, on 

 the other hand, suppressed. In certain cases also, as in adipose tissue, 

 bone, etc., other substances may be laid down in it. The various 

 kinds of tissue included under the heading of connective tissues are : 

 Areolar or Sub-cutaneous, Fibrous, Elastic, Adipose and Lymphoid 

 tissues, and Bone and Cartilage. By some authorities Blood and 

 Lymph are also regarded as being highly specialised varieties of 

 connective tissue. 



