192 FUNCTIONS OF THE BRAIN. 



Each neurone sends an enormous number of dendrites into the ganglion, 

 mingling with the central ends of the sensory fibers, and a large number of fibers 

 through the branchial nerve to the anterior and posterior branchial muscles. 

 Isolation of these centers, by cutting the cord on both the anterior and pos- 

 terior sides of the ganglion, does not destroy the action of the corresponding 

 appendage. 



The hypobranchial muscle extends diagonally forward from the tendinous 

 stigmata, or hollow infoldings at the base of the appendages, to the haemal surface 

 of the carapace. (Fig. 77.) It serves to flex the abdomen, but primarily to draw 

 the bases of the appendages forward and haemally, thus expanding the chamber 

 between the roots of the appendages, and drawing the water from the sides through 

 the gill leaves. 



It is innervated by a large, longitudinal nerve, formed by the union of seven 

 segmental nerves, one from the haemal nerve of the opercular segment and one 

 from each of the six following haemal nerves. Each segmental bundle of nerve 

 fibers takes its origin from a cluster of neurones located on the opposite side of 

 the next preceding ganglion, close to the reflex center for that appendage. (Figs. 

 59 and 60.) 



Respiratory Reflexes. — No final conclusion, as to the sources of the respi- 

 ratory impulses can be reached till the action of this nerve and muscle has been 

 experimentally demonstrated. It is clear that sectioning the cord at one or more 

 points would not be likely to greatly modify the action of the hypobranchial 

 muscle, as it would still receive nerves from the ganglia in front of and behind 

 the cuts. This point has been overlooked by Miss Hyde and has not been suffi- 

 ciently covered by our own experiments. 



It seems probable that the contraction of the muscle as a whole may be 

 induced by impulses coming from one or more neuromeres through the roots of 

 the anterior, or haemal, nerves, and as such a contraction would affect all the gills 

 at the same time it would tend to unify their action and thus materially aid the 

 linear coordination of the respiratory rhythm. 



The location of the common center controlling the whole series of branchial re- 

 flexes could not be determined, but judging from the forward displacement of the 

 motor cells and of the central ends of the accompanying sensory fibers, it probably 

 Hes, in part, at any rate, in the vagus neuromeres. This conclusion is strengthened 

 by the fact that the destruction of the vagus neuromeres materially modifies the 

 respiratory activities. It apparently lowers the threshold that inhibits the " cross 

 rubbing" or the normal respiratory mo\'ements, for if the vagus neuromeres are 

 destroyed, or separated from the cord, gentle stimulation of the isolated gills 

 with tactile, temperature, or chemical agents, starts the respiratory reflexes in them 

 much more readily than in those not so isolated. Moreover, the forced "yawn- 

 ing" of the gills and the swimming movements, which represent a modified re- 

 spiratory movement, disappear in those gills that are not directly connected with 

 the vagus region. There is also a striking difference in the rhythm, range, rate. 



