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



part of a valve. Hence the blood flows freely into the heart 

 from this membranous cavity or sinus, as I have termed it.* 

 By the contraction of the heart it is forced onwards through 

 the aorta and arteries, and eventually reaches the different 

 organs of the body. When the heart contracts there is a ten- 

 dency to expel the blood in two directions — forwards through 

 the arteries, and backwards into the sinus — but here the valve 

 comes into operation, and by preventing the backward flow, 

 allows the circulatory organ to employ all its power in the 

 propulsion of the fluid. 



The heart is a thin muscular bag, of a triangular or rather 

 a pear-shaped outline, and a delicate flesh-like colour. It is 

 situate in the region of the thorax, is surrounded by the peri- 

 cardial gland, and is bounded above and below by transparent 

 folds of tissue. It lies in an oblique plane, its apex pointing 

 backward and to the right, and its base in the opposite direc- 

 tion, and measures about a quarter of an inch in length, and 

 one-sixth or thereabouts in width. The heart has been incor- 

 rectly described as consisting of two cavities — an auricle and 

 ventricle. It is not divided by a partition of any kind, and is 

 to all intents and purposes a simple sac, composed of numerous 

 interlacing muscular fibres of the unstriped variety ; these are 

 filled with long spindle-shaped endoplasts,t containing clear 

 well-marked nuclei. If we examine the inner surface of the 

 heart with an ordinary pocket lens we may perceive a very 

 peculiar muscular arrangement. From two centres situate in 

 the lateral walls of the organ, sets of muscular chords pass out 

 in a radiating manner and ultimately become continuous with 

 the ordinary fibi'es ; in this way they form two stellate eleva- 

 tions, which possibly servo the same purpose as the " carnca' 

 columnce" of the heart of man and mammalia. 



The pulsations of the heart amount to about twenty in the 

 minute, each contraction being followed by a dilatation, and 

 then a period of repose occurring, during which the mem- 

 branous sinus is being gradually filled. When a dilatation 

 occurs, the sinus, owing' to the tendency to form a vacuum, is 

 instantly emptied, then a contraction following, the blood is 

 sent into the arteries, the sinus is refilled, and everything goes 

 on as before. 



The arterial system, or series of tubes which conveys the 

 bloody 'mm tho heart to the body generally, consists of the aorta, 

 with its branches and their uumerous divisions. The aorta 

 arises from the apex of the heart, and after attaining a length 

 of one-sixth of an inch divides into two trunks, each of which 

 has a calibre of about -'-jh of an inch; these remain in company 



* Vide Quarterly Journal of Microscopical Science, loo, tit, 

 f CVll-i of the older writers. 



