The Lungs, Heart, and Blood-vessels of the Slug. 105 



THE LUNGS, HEART, AND BLOOD-VESSELS OE 



THE SLUG. 



BY HENRY LAWSON, M.D., 

 Professor of Physiology in Queen's College, Birmingham. 

 (With a Tinted Plate.) 



We all know that man has lungs and a heart, and that the blood 

 circulates in his body, but there are many of us who would be 

 very much astonished to learn that as low an animal as the slug 

 has a heart and lungs also, and that its blood circulates through 

 arteries and veins, and is propelled through the former by the 

 pulsations of the heart. 



I hope to show in the present article that the mollusk in 

 question has all the organs to which I have alluded, and to 

 explain to you the steps by which you may arrive at the know- 

 ledge on your own account, should you be inclined to exhibit 

 scepticism regarding my statements and assertions. It is very 

 interesting to be informed that one creature possesses one 

 gland, and that another is known to have another, but it is vastly 

 more satisfactory to be able to take one's scalpel and forceps in 

 hand, and demonstrate for one's self the accuracy or it may be 

 the falsity of an author's statements. For although an anatomist, 

 well known for his original researches, may pledge himself to 

 some particular assertion, it by no means follows as a logical 

 conclusion that that assertion is true. The most skilled in ob- 

 serving natural objects may be occasionally deceived, or may be 

 so blinded, owing to some foregone deduction, as to see only 

 those things which they had anticipated, to the exclusion of 

 others as important but which do not touch immediately on their 

 darling speculations. Hence, as I have just mentioned, I pro- 

 pose pointing out to you the mode which you must pursue in 

 order to satisfy yourself regarding the truthfulness of the ob- 

 servations which I have here committed to paper. 



We have to study two great systems in the economy of the 

 slug — the respiratory and circulatory ; and as facts are of very 

 little value unless we associate them with ideas, let us ask what 

 is the use of a respiratory system ? The object of a lung is 

 to expose the blood freely to the influence of the air. The 

 function consists in certain operations going on simultaneously 

 in the blood and atmosphere, and which result in the purification 

 of the former. 



The blood has two important duties to discharge. 



1st. It absorbs from the stomach the valuable portions of 

 the food, which it carries to the different tissues in order to 

 repair them. 



2nd. It abstracts from the tissues the refuse matter of the 



