On the other hand in 35 experiments, when fleas had access to the spaces, plague infections were the rule. Again, guinea pigs in cages which were suspended only two inches above a plague flea infected floor, became infected, but other animals, which were suspended so high that the fleas could not jump up to them, remained well. Two cages, each containing a monkey, were placed in a plague flea infected room. One was surrounded with a protecting zone of 6 inches of “tanglefoot” fly paper, this being the limit of the distance a flea can jump, while the other cage was not so protected. The monkey in the cage without the sticky paper contracted plague while the second monkey remained well. It is only when there is a great incidence of plague among rats that we have outbreaks of bubonic plague in man, and it has been noted that the greater the epizootic, the more heavily infected was the blood of the sick rats with the plague bacilli. A flea with a stomach capacity of about ½ c.mm. could take in several thousand plague bacilli in a feeding on a rat whose blood was teeming with bacilli. The blood of a rat dying with plague may contain as many as fifty million bacilli to the cc. Human blood rarely contains more than a million to the cc. There is a multiplication of the organisms in the flea, so that when it defecates, thousands of plague bacilli are deposited near the puncture wound made by the flea when subsequently feeding on a man. The infected faeces are rubbed into the wound by the man in scratching the site of the bite, so that we have here an instance of a contaminative method of infection as contrasted with the inoculative method by the mosquito in malaria. [Illustration: FIG. 66.—1, _Ctenocephalus felis_. 2, _Ceratophyllus fasciatus_. 3, _Hoplopsyllus anomalus_. 4, _Ctenopsylla musculi_. 5, _Xenopsylla cheopis_. 6, _Pulex irritans_. 7, Internal anatomy of flea. (After Fox.) (_a_) Maxillary palpus; (_a_-1) epipharynx; (_a_-2) mandible; (_a_-3) labial palpi; (_a_-4) maxillae; (_a_-5) basal elements of rostrum and mandibles; (_b_) salivary pump; (_c_) hypopharynx; (_d_) aspiratory pharynx; (_e_) muscles of the aspiratory pharynx; (_f_) eye; (_g_) oesophageal ganglia (brain); (_h_) thoracic ganglia; (_i_) oesophagus; (_j_) salivary duct; (_k_) gizzard; (_l_) salivary gland; (_m_) stomach; (_n_) aorta; (_o_) ovaries; (_p_) malpighian tubules; (_q_) pygidium; (_r_) rectum showing rectal glands; (_s_) anus; (_t_) intestines; (_u_) bursa copulatrix; (_u_-1) ductus obturatorius (blind duct); (_v_) receptaculi seminis or spermatheca; (_w_) ducts of spermatheca; (_x_) vagina; (_y_) uterus; (_z_) abdominal ganglia.] Bacot and Martin have shown that while infection may take place as the result of the faeces being rubbed into the wound made at the time of feeding yet the ordinary way is probably by regurgitation from the oesophagus of the flea at the time of feeding.