590 INTEGUMENTARY SYSTEM 



As indicated above, the distribution of nipples and mammary glands along 

 the ventral abdominal wall varies greatly in different mammalian species. In 

 lemurs and fruit bats, the mammary glands are developed in the axillary region; 

 in the human and in primates, they are pectoral; in the cat, they are best 

 developed in the pecto-abdominal area; in the dog and pig, they are mainly 

 well developed in the abdominal and inguinal areas; in the cow and horse, 

 inguinal nipples only appear; and in whales, the mammary glands are located 

 near the external genitals. 



The development of supernumerary mammary glands, i.e., hypermastia, is 

 rare, but the formation of extra nipples, i.e., hyperthelia, is common in both 

 male and female. In female mammals, such as the bitch, it is not uncommon 

 for the breasts to remain in an undeveloped condition in the pectoral area, 

 whereas those in the inguinal and abdominal areas are normal. When the 

 mammary glands continue in an undeveloped or regressed state as, for ex- 

 ample, in the anterior pectoral region of the bitch, the condition is known as 

 micromastia. On the other hand, the abnormal development of the mammary 

 glands to an abnormal size is known as macromastia. The latter condition 

 often is found in cattle and occasionally in the bitch and human. 



C. Coloration and Pigmentation of the Vertebrate Skin and 

 Accessory Structures 



1. Factors Concerned with Skin Color 



The color of the skin and its accessory structures is dependent upon five 

 main factors: 



( 1 ) the color of the skin itself, 



(2) its opacity or translucency, 



(3) the presence of pigment granules and special, pigment-bearing cells, 



(4) the capillary bed of blood vessels which lies within the dermal portion 

 of the skin, and 



(5) the color of the accessory structures. 



The color of the skin itself varies considerably in different species, but it 

 tends to be slightly yellow, resulting from the presence of fatty tissue, fat 

 droplets, and constitutent, connective-tissue fibers in the dermis. The prop- 

 erty of opacity or translucency is an important factor for upon it depends 

 transmission of light waves through the skin from deeper lying structures, 

 such as blood vessels, pigment droplets, pigment-bearing cells, etc. The pres- 

 ence of definite types of pigment granules within or between the cells of the 

 epidermis and dermis determines the course and kind of light waves which 

 are reflected. The richness or paucity of blood vessels, ramifying through the 

 dermal area, also affects the skin's color in many instances. 



The color of the accessory structures, particularly the structures derived 



