j 2 HISTOLOGY 



retain their natural color in stained specimens. They are said to consist 

 of "melanin," which represents an ill-defined group of substances, some of 

 which are haemoglobin derivatives. In the lung, inhaled soot is taken 

 into the protoplasm of certain cells which thus become pigmented with 

 extraneous material. Pigment granules are widely distributed, and 

 may be found in the liver, spleen, heart, brain, and other organs. 



In certain situations, pigment is extensively developed in branched 

 connective tissue cells such as are shown in Fig. 57, A. In man these 

 are of limited occurrence, being found near the eye, and in the pia 

 mater, especially under the medulla oblongata and upper portion of the 



spinal cord. Weidenreich con- 



-*&! -^T^^ siders that this represents the 



remains of a general pigmented 

 sheath for the entire nervous 

 system. In lower vertebrates 

 branching pigment cells are 

 often abundant in the sub- 

 cutaneous tissue, and changes 



FIG. 57. A, Two pigment cells from the deep, peripheral __1__ ,.,,-1, oc nrrnr in frr>a<s 



part of the cornea of the rabbit. B, Pigmented m COlOr, SUCn aS OCCUr in irOgS, 

 epithelium from the conjunctiva of the guinea-pig. j , 4.-L- r ^-f ^^OI'^T-, /-. 



The pigment is chiefly in the basal layer. are due tO the extension OT 



retraction of these processes. 



Such pigmented connective tissue cells are called chromatophores or 

 chromatocytes. But in the human skin the pigment granules are in the 

 epidermis, chiefly in the basal layers. In the stratified epithelium of the 

 conjunctiva of the eye, toward the cornea, numerous pigment granules 

 are found in the basal layers, and scattered groups occur also in the outer 

 layers, as shown in Fig. 57, B. Pigment in this situation occurs fre- 

 quently in the Caucasian race, and regularly in the other human races. 

 Simple epithelium may be densely pigmented, as in the external epithe- 

 lium of the retina. Thus it is seen that pigment cells are by no means 

 limited to connective tissue. 



ADIPOSE TISSUE. 



If in a freshly killed animal a loop of intestine is drawn out of the 

 abdominal cavity, the blood vessels ramifying in its mesentery will be 

 seen to be imbedded in a band of fat, which branches when the vessels 

 branch, and diminishes in width toward the intestine as the vessels 

 become small. The close relation between the distribution of fat and the 

 course of the vessels is notable also in sections. Fat cells occur in groups 

 or lobules around the vessels, and are found, with few exceptions, wherever 

 there is loose connective tissue. They may also occur singly, as in some 

 parts of the denser connective tissue of the breast. 



