§3.111 CONTROL OF MUSCLES 57 



in the case of gastrin, or chemical, as in that of secretin (§ 4.1 1 1) ; 

 but it is usually nervous (§ 4.32). Few are under the control of any 

 other hormones, and this is probably a question of speed. Hormone 

 action is slower than nerve action; and, whereas the delay due to 

 one hormone may not be significant for the effectors in question, a 

 chain of two hormones might well be so. 



The hormones dealt with in the following sections are shown in 

 a series of tables, where they are arranged according to their actions. 

 The hormones of vertebrates have names which are widely 

 accepted and are known to occur in a variety of animals; the 

 example quoted is usually the one described in the text, but in no 

 way implies that the hormone is limited to the genus named. 

 Hormones of invertebrates for the most part have no names and 

 can therefore only be referred to by the organ from which they 

 are secreted. If they have a name, or an abbreviation that is used 

 in the text, this is given in a separate column. Each invertebrate 

 example in the tables is also referred to in the text ; it is often the 

 only example from which the hormone has so far been identified. 



3.1 Control of muscles 

 Muscles can be grouped according to their functions or their 

 histology, and there is some evidence that all types can be influ- 

 enced by hormones. It will be convenient to take the involuntary 

 muscles of the viscera and heart first, because these are the most 

 commonly subject to hormone control and react similarly, although 

 most visceral muscle of vertebrates is smooth, or unstriated, and 

 that of arthropods and of the hearts in both phyla is striated. 



3.11 VISCERAL MUSCLE 



In nearly all cases the action of hormones is to stimulate both 

 the rate and amplitude of the contraction of visceral muscle ; only 

 rarely is a hormone known to inhibit muscle action (Table 7). 



3.111 Heart muscle 



CRUSTACEA. Control of heart muscle by a hormone has been 

 shown experimentally in the crab. Cancer pagtinis, and the lobster, 

 Homarus vulgaris. Extracts of the neurosecretory "pericardial 

 organs" (§2.112) added to fluid perfusing isolated hearts, 



