FORCES CONCERNED IN MO VEMENT OF L YMPII 301 
could be largely increased by passive flexion and extension of the limbs. 
We must therefore loot upon the entire muscular system as one of 
the chief sources of the energy for maintaining the lymphatic circula- 
tion, especially as the presence of valves in the lymphatics converts 
every muscular contraction which may press on the vessels into a 
driving force. 
We have finally to consider the effect of changes in the calibre of 
the lymphatics themselves on the onward flow of lymph. In the frog 
(and in other amphibia, and also in Saur&psida) the lymph circulation 
is maintained by special contractile cavities called lymph hearts, situated 
in pairs, an anterior pair beneath the scapulae, and a posterior pair in 
the ileo-coccygeal space. 
The chief points with regard to the normal anatomy and physiology of the 
batrachian lymph hearts have been summed up as follows, by J. Priestley : : — 
1. The hearts are muscular sacs, the fibres of which branch and freely 
anastomose and are transversely striated. Their walls are penetrated by 
medullated and non-meduUated nerve fibres, and small nerve ganglia are 
situated in the neighbourhood of the hearts, but no ganglion cells 
have as yet been recognised amidst the muscular fibres. They collect the 
lymph from more or less extensive lymphatic regions, and force it past valves 
into large veins, the anterior pair of hearts into branches of the jugular, the 
posterior pair into branches of the ischiatic vein. They are supplied by 
nerves from the spinal cord, the anterior pair by the second, the posterior pair 
by the tenth spinal nerve. 
2. The hearts exhibit throughout life a pulsation with a mean rate of 
sixty to seventy a minute. It is, however, not continuously regular, being 
interrupted by pauses, and by periods of great acceleration. The pauses some- 
times follow movements on the part of the animal, but often they cannot be 
set down to any definite cause. After such pauses the pulsations begin as 
twitches before falling into beats of normal fulness. The periods of acceleration 
also seem to be determined, for the most part, by movements of the animal. 
3. The hearts are governed by cerebro-spinal centres — motor and inhibitory. 
The motor centres are situated in the spinal cord, those for the anterior pair 
opposite the third, and those for the posterior pair opposite the sixth 
vertebra. They transmit their impulses down the appropriate spinal nerves 
of their own side of the body ; and each is independent of the rest. They 
originate the normal rhythm of the hearts ; and their action, whatever its exact 
nature, is automatic, or not due directly to afferent stimuli ; hence no change 
in the lymph current traversing the hearts can alter their rhythm. The 
inhibitory centre is situated in the encephalon, in the optic lobes ; it is 
constantly in action. 
4. These centres are in connection with afferent nerves. Strong stimuli, 
applied to the blood heart or to the abdominal viscera, lead to inhibition of the 
heart beats, if the upper centre is intact ; while strong sensory stimuli applied 
to the skin may inhibit the lymph hearts whether the upper centre is 
present or not. 
5. But though governed by the above centres, the lymph hearts seem 
capable of an irregular pulsation when separated from them. Such pulsation 
consists of flickers and indefinite confused twitchings for the most part, 
which, when the heart is vigorous, harmonise occasionally to full beats. The 
nature of these movements is still doubtful. The most that can be said about 
them is that they are probably not solely muscular, since curari abolishes them. 
1 Journ. Physiol., Cambridge and London, 1879, vol. i. p. 1. Cf. also the account by v. 
"Wittich in Hermann's :1 Handbuch," Bd. v. (2) S. 325, where full references to the liter- 
ature of the subject are given. 
