. Evolution and disease . IlG. 87.—The arm oi llapaloiiur i^iiscus, sliowing itsbrachial mam nice. way it would be difficult for it to use the inguinal teats,if any existed, whilst it is within easy access of thepectoral pair. It seems to me reasonable to supposethat this habit of carrying the young has led to thepectoral teats being most used, and their gradualenlargement from increased use would slowly bringabout the suppression of the little used inguinal the nipples are present in the male, and havea disposition corresponding to that of the female. A review of the arrangement o
. Evolution and disease . IlG. 87.—The arm oi llapaloiiur i^iiscus, sliowing itsbrachial mam nice. way it would be difficult for it to use the inguinal teats,if any existed, whilst it is within easy access of thepectoral pair. It seems to me reasonable to supposethat this habit of carrying the young has led to thepectoral teats being most used, and their gradualenlargement from increased use would slowly bringabout the suppression of the little used inguinal the nipples are present in the male, and havea disposition corresponding to that of the female. A review of the arrangement of the milk glands inmammals generally indicates that primitively they were 166 EVOLUTION AND DISEASE. arranged in two rows, extending along the ventralaspect of the trunk in both sexes ; and in situation theycorrespond to the course of the deep and superiorepigastric arteries, an extraordinary anastomosis effect-. FiG. 88.—Lemur macaco and young. (After Sclater.) ing a free communication between the subclavianarteries at the anterior part of the thorax and the iliacsystem of arteries at the groin. This vascular arrange-ment is peculiar to mammals, and has direct relationwith the mammary glands. ATAVISM. 167 When the mammae, or nipples, are increased beyondthe number normal to a given mammal, they are saidto be accessory or supernumerary, and occur in threeforms :— 1. As nipples or teats. 2. As mammae, furnished with teats. 3. As nipple-less mammae. Such mammae, or nipples, may be due to atavism, orarise by dichotomy of a normal gland, or occur as aspontaneous variation or sport. Notwithstanding the scanty evidence at the disposalof Darwin when he wrote The Descent of Man, heregarded supernumerary mammary glands as reversions,although he considered the opinion weakened by theiroccurrence on the thigh and back. Since then ourknowledge on this subject has increased, and has hadthe effect of st
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