Epithelial Structures. 



11 



adult condition the gland is found separated from the epithelium from 

 which it was originally formed. This condition is represented by the 

 thyreoid and thymus glands of the neck and thorax respectively. In 

 other cases the secreting element is a single cell, the latter thus repre- 

 senting a unicellular gland lying directly in the general layer. The 

 mucus-secreting goblet cells of the intestinal wall are structures of this 

 nature. 



The majority of multicellular glands conform to one of two types, 

 namely, the tubular gland, in which the secreting portions are of uniform 

 calibre, and the acinous or alveolar gland, in which the secreting portions 

 are sacculated (Fig. 3, A). Both types occur in simple, little branched 

 and greatly branched conditions. 



Cutaneous glands of two types are commonly present in mammals in 

 association with the hairs, namely, sudoriparous or sweat-glands, which 

 are glands of the tubular type, and seba- 

 ceous glands, which are of the acinous 

 type. In the rabbit, glands are absent 

 from the general surface, but are found in 

 special situations, as, for example, in con- 

 nection with the hair follicles of the lips, 

 the internal surface of the ear, and the 

 external genital organs. The inguinal 

 glands comprise both tubular and acinous 

 portions. The mammary glands of the 

 female are greatly modified cutaneous 

 glands of an acinous type. 



Apart from the mucus-secreting cells of 

 the general epithelium, the glands of the 

 alimentary canal comprise the important 

 but less elaborated glands of the wall, such 

 as the gastric glands of the wall of the 

 stomach, and the greatly elaborated, out- 

 standing glands which lie beyond the wall 

 and are connected with the interior of the 

 canal only through their ducts. The latter 

 comprise the oral glands, the liver, and the 

 pancreas. The oral glands include chiefly 

 the submaxillary, parotid, sublingual, and 

 infraorbital glands — conspicuous structures 

 in the dissection of the surrounding por- 

 tions of the head and neck. Their ducts 

 communicate with the cavity of the mouth. 



There is a close association between the epithelia of the surface of the 

 body and the nervous tissues, arising from the circumstance that they are 

 derivatives of a common embryonic layer, the ectoderm. In the adult 

 we may distinguish as sensory epithelia special aggregations of cells lying 

 either in a deep or superficial position, and associated more or less closely 

 with the central nervous system. They comprise the olfactory epithelium 

 of the nasal cavity, some of the cells of which are true nerve cells, the 



Fig. 3. A, Diagram of a multi- 

 cellular gland: al., alveolar type; d., 

 duct; ep., general epithelium; t., 

 tubular type. B , The pancreatic duct 

 and associated portion of the duo- 

 denum in the rabbit. 



